Merge branch 'checkpointing-fix' into 'master'

small fix for CheckpointFunction's backward() method when some args may be NoneType

See merge request ADLR/megatron-lm!92

megatron/mpu/random.py

@@ -234,7 +234,9 @@ class CheckpointFunction(torch.autograd.Function):
         if isinstance(outputs, torch.Tensor):
             outputs = (outputs,)
         torch.autograd.backward(outputs, args)
-        return (None,) + tuple(inp.grad for inp in detached_inputs)
+        grads = tuple(inp.grad if isinstance(inp, torch.Tensor) else inp
+                      for inp in detached_inputs)
+        return (None,) + grads
 
 
 def checkpoint(function, *args):