Cherry-pick [2.7] pin fastdigest==0.4.0 due to API changes (#4217)

[YuanTingHsieh]

Summary

• pin fastdigest to 0.4.0 in setup.cfg for 2.7 test environments
• keep 2.7 on the existing TDigest API by reverting temporary compatibility-layer code changes in quantile runtime/tests
• remove temporary user-facing example API additions so example/job interfaces remain unchanged
• update docs/READMEs/tutorial notebooks to explicitly state 2.7 quantile workflows should use fastdigest==0.4.0

Test plan

• python -m py_compile nvflare/app_opt/statistics/df/df_core_statistics.py nvflare/app_opt/statistics/quantile_stats.py tests/unit_test/app_common/statistics/quantile_test.py
• python -m py_compile examples/advanced/federated-statistics/df_stats/client.py examples/advanced/federated-statistics/df_stats/job.py examples/hello-world/hello-tabular-stats/client.py examples/hello-world/hello-tabular-stats/job.py
• pytest -q tests/unit_test/app_common/statistics/quantile_test.py (blocked locally: current macOS env segfaults on import numpy)
• Linux CI / non-mac validation for quantile tests with fastdigest==0.4.0

Notes

• This PR intentionally avoids user-facing API changes on 2.7.
• TDigest API migration for newer fastdigest versions will be handled later on main.

Fixes # .

Description

A few sentences describing the changes proposed in this pull request.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[greptile-apps]

Greptile Summary

This PR pins fastdigest to 0.4.0 in the test extras of setup.cfg and propagates that pinned version requirement across all user-facing documentation (RST docs, READMEs, and Jupyter notebooks), keeping the 2.7 branch stable against the breaking TDigest API changes introduced in newer fastdigest releases.

Key changes:

• setup.cfg: Adds fastdigest==0.4.0; python_version < "3.14" to the [options.extras_require] test group, but not to test_mac — creating a potential gap if Mac quantile tests are ever unblocked.
• All documentation touchpoints (federated_statistics_overview.rst, index.rst, two READMEs, two Jupyter notebooks) are updated consistently to direct users to pip install fastdigest==0.4.0.
• The python_version < "3.14" environment marker in setup.cfg silently excludes the pin on Python 3.14+; this is worth confirming is intentional (e.g., because fastdigest==0.4.0 does not build on that interpreter).

Confidence Score: 4/5

• Safe to merge with minor follow-up: the test_mac extras omission and the python_version < "3.14" guard should be verified as intentional.
• All changes are documentation updates and a single dependency pin. No runtime or user-facing API is modified. The only potential issue is that fastdigest==0.4.0 is added to test but not test_mac, which could silently break Mac quantile tests once the unrelated numpy blocker is resolved. The python_version < "3.14" guard is also worth confirming as intentional.
• setup.cfg — verify that the test_mac omission and python_version < "3.14" guard are intentional.

Important Files Changed

Filename	Overview
setup.cfg	Pins fastdigest==0.4.0 (with a python_version < "3.14" guard) to the test extras only; test_mac is not updated, creating a potential inconsistency.
docs/examples/federated_statistics_overview.rst	Adds a pip install fastdigest==0.4.0 code block and a note that the newer TDigest API will be adopted in a later release; straightforward documentation update.
docs/hello-world/hello-tabular-stats/index.rst	Updates the fastdigest install instruction to specify the exact pinned version fastdigest==0.4.0.
examples/advanced/federated-statistics/df_stats/README.md	Adds a pip install fastdigest==0.4.0 bash code block under the quantile description section; clean, correct change.
examples/hello-world/hello-tabular-stats/README.md	Updates install instruction from a generic fastdigest mention to the pinned fastdigest==0.4.0.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["pip install nvflare test"] --> B{"python_version < 3.14?"}
    B -- Yes --> C["fastdigest==0.4.0 installed"]
    B -- No --> D["fastdigest NOT installed"]
    C --> E["Quantile tests pass on Linux CI"]
    D --> F["Quantile tests fail on Python 3.14+"]

    G["pip install nvflare test_mac"] --> H["fastdigest NOT in test_mac extras"]
    H --> I["Quantile tests may fail on Mac CI"]

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_{Last reviewed commit: 01bf045}

[greptile-apps]

User-facing phrasing references internal branch concept

The phrase "planned to be adopted in a later branch" uses Git branching terminology that is internal to the project and may confuse end users. Consider replacing it with version-oriented language such as "a future release" to keep the documentation audience-appropriate.

Suggested change

	Newer ``fastdigest`` releases changed the TDigest API and are planned to be adopted in a later branch.
	Newer ``fastdigest`` releases changed the TDigest API and are planned to be adopted in a future release.

_{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}

[Copilot]

Pull request overview

Pins fastdigest to a specific version for the 2.7 branch to avoid TDigest API drift, and updates user-facing docs/tutorials to consistently instruct fastdigest==0.4.0 for quantile workflows.

Changes:

• Add fastdigest==0.4.0 to the test extras in setup.cfg.
• Update multiple READMEs/docs/notebooks to explicitly pin fastdigest==0.4.0 for quantile examples.
• Add a pinned-install snippet to the advanced federated-statistics README.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
setup.cfg	Pins fastdigest==0.4.0 under test extras to keep test envs on the older TDigest API.
examples/tutorials/.../federated_statistics_with_tabular_data.ipynb	Updates tutorial text to instruct installing fastdigest==0.4.0 for quantiles.
examples/hello-world/hello-tabular-stats/df_stats.ipynb	Updates notebook instructions to pin fastdigest==0.4.0.
examples/hello-world/hello-tabular-stats/README.md	Updates README to pin fastdigest==0.4.0.
examples/advanced/federated-statistics/df_stats/README.md	Adds explicit “install and pin” snippet for fastdigest==0.4.0.
docs/hello-world/hello-tabular-stats/index.rst	Updates docs to pin fastdigest==0.4.0.
docs/examples/federated_statistics_overview.rst	Adds guidance to pin fastdigest==0.4.0 and notes API changes in newer releases.

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[Copilot]

The updated sidebar sentence has grammatical issues (e.g., "the fastdigest not included") and double spacing after the period. Please revise to a complete, capitalized sentence (e.g., "fastdigest is not included in requirements.txt").

Suggested change

	"> If you intend to calculate quantiles, install `fastdigest==0.4.0`. the fastdigest not included in the requirements.txt file. If you are not calculating quantiles, you can skip this step.\n",
	"> If you intend to calculate quantiles, install `fastdigest==0.4.0`. The `fastdigest` package is not included in the `requirements.txt` file. If you are not calculating quantiles, you can skip this step.\n",