[rocm7.0_internal_testing] skip test_all_gather_extensions_train_parity if world_size less then 2

[dnikolaev-amd]

Base class forces to use world_size=2 even for 1 GPU. Then NCCL fails with errors:

ncclInvalidUsage: This usually reflects invalid usage of NCCL library.
Duplicate GPU detected : rank 1 and rank 0 both on CUDA device c000
Duplicate GPU detected : rank 0 and rank 1 both on CUDA device c000

This PR will skip FSDP tests if world_size > torch.cuda.device_count()

HIP_VISIBLE_DEVICES=0 pytest -v distributed/_composable/fsdp/test_fully_shard_extensions.py::TestFullyShardAllGatherExtensionsMultiProcess::test_all_gather_extensions_train_parity

dist init r=0, world=2
dist init r=1, world=2
SKIPPED [15.5507s] (Need at least 2 CUDA devices)

Fixes SWDEV-535767

[rocm-repo-management-api]

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[pragupta]

We should also try to upstream this and hope upstream folks are okay with this fix.