fix GradBucket.is_last() logic (#63768)

fix GradBucket.is_last() logic (#63768)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63768

passed number of buckets to GradBucket constructor, to check if index is equal to num_buckets - 1 in the .is_last() function.

Test Plan:
buck test mode/dev-nosan //caffe2/test/distributed/algorithms/ddp_comm_hooks:test_ddp_hooks

test output: https://www.internalfb.com/intern/testinfra/testconsole/testrun/8162774375985873/

Reviewed By: SciPioneer, mrshenli

Differential Revision: D30455913

fbshipit-source-id: 8c67ca69cbf191d6e189e09248407eb167bb24b6