[DDP Communication Hook] Rename 4 Methods of GradBucket Class
Summary:
1. getPerParameterTensors -> getGradients
2. getModelParamsForBucket -> getParameters
3. isTheLastBucketToAllreduce -> IsLast
Test Plan:
Test results for "buck test mode/dev-nosan caffe2/test/distributed:c10d":
https://pxl.cl/1Mrq8
Test results for "buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork":
https://pxl.cl/1MrtP
Reviewed By: SciPioneer
Differential Revision: D30076436
fbshipit-source-id: 0bd1e410186a318ea6328f4c1e830ea5632f8a47
