PCAM: fix DataLoader pickling error by avoiding module on self

[nehdiii]

PCAM: lazy-import h5py via _get_h5py to keep dataset picklable

Fixes: #9195

Problem

• PCAM previously stored the h5py module on the dataset instance, making it unpicklable.
• DataLoader(num_workers>0) / spawn-based DDP pickle the dataset, causing TypeError: cannot pickle 'module' object.

Change

• Introduce _get_h5py() helper that imports h5py on demand with a clear error message.
• Use h5py = _get_h5py() in __len__ and __getitem__ (and any other call sites) instead of self.h5py.
• No import at __init__; no module stored on self. Error text for missing h5py remains unchanged.

Tests

1. test_update_PCAM.py

   • Adds PCAMTestCase (inherits ImageDatasetTestCase).
   • inject_fake_data(...) writes tiny HDF5 files for each split (train, val, test) with datasets x and y.
   • Requires h5py. Exercises core dataset behaviors on synthetic data (offline).

2. test_update_PCAM_multiprocessing.py

   • Spawn-based DDP smoke test (torch.multiprocessing.spawn with world_size=2, backend gloo).
   • Uses DistributedSampler and DataLoader(num_workers=2, persistent_workers=True).
   • Iterates a few batches and performs an all_reduce sanity check to ensure workers progress without pickling errors.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9200

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[deo-abhijit]

Hey @nehdiii , I think we can remove the test/test_update_pcam.py file as we are doing the same at https://github.com/pytorch/vision/blob/ce5b26a9284d119fe9fc63d0552abb35e0ba802a/test/test_datasets.py#L2858

Also, since we don’t currently have multiprocessing tests in the suite, the dedicated multiprocessing test file could be removed as well.

Other than that, this PR looks very crisp.