Avoid importing apex transformer automatically and make error messages more clear when apex.transformer is explicitly called on unsupported platform

Author: nWEIdia

apex/__init__.py

@@ -24,7 +24,6 @@
 # load time) the error message is timely and visible.
 from . import optimizers
 from . import normalization
-from . import transformer
 
 
 # Logging utilities for apex.transformer module

apex/transformer/utils.py

@@ -8,6 +8,7 @@
 # The following 4 lines are for backward comparability with
 # older PyTorch.
 if "all_gather_into_tensor" not in dir(torch.distributed):
+    assert torch.distributed.is_available(), "PyTorch Distributed is Not available or Disabled."
     torch.distributed.all_gather_into_tensor = torch.distributed._all_gather_base
 
 def ensure_divisibility(numerator, denominator):