Update nightly version to 2.15.0 - #8509

The 2.14 branch has been cut and `pytorch/pytorch` `main` is now on
`2.15.0a0`, so trunk nightlies are `2.15.0.dev*`. `CURRENT_NIGHTLY_VERSION`
was left behind at 2.14.0.

This is the separate follow-up the go-live skill describes: the post-branch-cut
automation advances `CURRENT_CANDIDATE_VERSION` (done in pytorch#8497) but
deliberately does not touch `CURRENT_NIGHTLY_VERSION`.

After this the three constants describe what each channel actually serves:

    CURRENT_NIGHTLY_VERSION   = "2.15.0"   # trunk nightlies
    CURRENT_CANDIDATE_VERSION = "2.14.0"   # 2.14 RCs on the test channel
    CURRENT_STABLE_VERSION    = "2.13.0"   # current GA

The only behavioural change is the nightly docker image tag in
`generate_docker_release_matrix.py`, which is built from this constant:

    before: ghcr.io/pytorch/pytorch-nightly:2.14.0.dev20260812-cuda12.6-cudnn9-runtime
    after:  ghcr.io/pytorch/pytorch-nightly:2.15.0.dev20260812-cuda12.6-cudnn9-runtime

which now matches the nightly wheels being published. The binary build matrix
is unaffected: `initialize_globals` derives `CURRENT_VERSION` from
`CURRENT_CANDIDATE_VERSION` on the test channel and `CURRENT_STABLE_VERSION`
everywhere else, so `CURRENT_NIGHTLY_VERSION` never reaches it.

It also means the next branch cut starts from the right value, since
`release-pytorch-post-branch-cut.yml` reads `CURRENT_NIGHTLY_VERSION` to set
`CURRENT_CANDIDATE_VERSION`.

Test plan:
- `python -m tools.tests.test_generate_binary_build_matrix` - 14 tests pass.
- `--update-reference-files` produces no asset changes.
- Binary build matrix is byte-identical (md5) before and after for
  {linux, linux-aarch64, windows, macos-arm64} x {nightly, test, release}.
- Docker release matrix nightly tag moves 2.14.0.dev -> 2.15.0.dev as above;
  the test and release channels are unchanged.

Authored with the assistance of Claude Code.