Add num_nodes() method to ClusterEnvironment

[crawfordxx]

Summary

• Adds an abstract num_nodes() method to the ClusterEnvironment base class so the number of nodes can be queried directly from the cluster environment
• Implements num_nodes() in all seven concrete subclasses (SLURM, TorchElastic, Lightning, Kubeflow, LSF, MPI, XLA), each deriving the value from the appropriate environment-specific source
• Adds unit tests for the new method across all testable environments

Motivation

Closes #7361. Currently num_nodes must be specified manually by the user, but in most cluster environments this information is already available (e.g., SLURM_NNODES, GROUP_WORLD_SIZE). This change allows the cluster environment to provide it automatically.

Implementation Details

Environment	Source
SLURMEnvironment	SLURM_NNODES env var (default: 1)
TorchElasticEnvironment	GROUP_WORLD_SIZE env var (default: 1)
LightningEnvironment	NUM_NODES env var (default: 1)
KubeflowEnvironment	NUM_NODES env var (default: world_size)
LSFEnvironment	Unique hosts in LSB_DJOB_RANKFILE
MPIEnvironment	Unique hostnames via MPI.COMM_WORLD.gather
XLAEnvironment	xr.host_count() (XLA >= 2.1) or HOST_WORLD_SIZE env var

Test plan

• Added dedicated test_num_nodes and test_num_nodes_default tests for SLURM, TorchElastic, Lightning, and Kubeflow environments
• Added inline assertion in existing test_attributes_from_environment_variables for SLURM and TorchElastic
• All 42 environment tests pass locally

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📚 Documentation preview 📚: https://pytorch-lightning--21638.org.readthedocs.build/en/21638/

[codecov]

Codecov Report

❌ Patch coverage is 64.70588% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 48%. Comparing base (0a60211) to head (487aa22).
⚠️ Report is 1 commits behind head on master.

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HEAD has 210 uploads less than BASE

Flag	BASE (0a60211)	HEAD (487aa22)
cpu	84	42
python	6	3
python3.10	6	3
lightning	30	15
python3.12	24	12
python3.12.7	18	9
python3.11	12	6
python3.13	18	9
pytorch2.1	6	0
pytest-full	42	0
pytorch2.3	3	0
pytorch2.8	6	0
pytorch_lightning	27	0
pytorch2.5.1	3	0
pytorch2.9	6	0
pytorch2.4.1	3	0
pytorch2.10	6	0
pytorch2.6	3	0
pytorch2.2.2	3	0
pytorch2.7	3	0

Additional details and impacted files

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- Coverage      87%      48%    -39%     
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  Files         270      267      -3     
  Lines       23930    23904     -26     
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- Hits        20709    11414   -9295     
- Misses       3221    12490   +9269