Update distopt API for coalesced NCCL calls
# What does this PR do ?

The Pytorch API for coalescing NCCL calls has changed in 2.1.0 (see https://github.com/pytorch/pytorch/pull/98793), which required some changes in the Apex wrapper functions (see https://github.com/NVIDIA/apex/pull/1663).

**Collection**: NLP

# Changelog 
- Use updated API for coalescing NCCL calls in distributed optimizer

# Usage
Set the optimizer to `distributed_fused_adam` in the config file.

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* PyTorch API change in https://github.com/pytorch/pytorch/pull/98793
* Apex API change in https://github.com/NVIDIA/apex/pull/1663