Exclude apex.transformer if distributed is not available

[Aidyn-A]

We need to gate against torch.distributed.is_available() check because otherwise it will fail on import:

$ python -c "import apex"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python3.12/dist-packages/apex/__init__.py", line 16, in <module>
    from . import transformer
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/__init__.py", line 4, in <module>
    from apex.transformer import pipeline_parallel
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/pipeline_parallel/__init__.py", line 1, in <module>
    from apex.transformer.pipeline_parallel.schedules import get_forward_backward_func
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/pipeline_parallel/schedules/__init__.py", line 3, in <module>
    from apex.transformer.pipeline_parallel.schedules.fwd_bwd_no_pipelining import (
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/pipeline_parallel/schedules/fwd_bwd_no_pipelining.py", line 10, in <module>
    from apex.transformer.pipeline_parallel.schedules.common import Batch
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/pipeline_parallel/schedules/common.py", line 9, in <module>
    from apex.transformer.pipeline_parallel.p2p_communication import FutureTensor
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/pipeline_parallel/p2p_communication.py", line 25, in <module>
    from apex.transformer.utils import split_tensor_into_1d_equal_chunks
  File "/usr/local/lib/python3.12/dist-packages/apex/transformer/utils.py", line 11, in <module>
    torch.distributed.all_gather_into_tensor = torch.distributed._all_gather_base
                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'torch.distributed' has no attribute '_all_gather_base'

cc @crcrpar