Enable torch.autocast with ZeRO

[tohtana]

DeepSpeed supports mixed precision training, but the behavior is different from torch.autocast. DeepSpeed maintains parameters and gradients both in FP32 and a lower precision (FP16/BF16) (NVIDIA Apex AMP style) and computes all modules in the lower precision while torch.autocast maintains parameters in FP32 but computes only certain operators in the lower precision.
This leads to differences in:

• performance: torch.autocast needs downcast in forward/backward
• memory usage: DeepSpeed needs more memory to keep copies of parameters and gradients in lower precision
• accuracy: torch.autocast has a list of modules that can safely be computed in lower precision. Some precision-sensitive operators (e.g. softmax) are computed in FP32.

To align DeepSpeed's behavior with torch.autocast when necessary, this PR adds the integration with torch.autocast with ZeRO. Here is an examples of the configuration.

"torch_autocast": {
  "enabled": true,
  "dtype": "bfloat16",
  "lower_precision_safe_modules": ["torch.nn.Linear", "torch.nn.Conv2d"]
}

Each configuration works as follows:

• enabled: Enable the integration with torch.autocast if this is set to True. You don't need to call torch.autocast in your code. The grad scaler is also applied in the DeepSpeed optimizer.
• dtype: lower precision dtype passed to torch.autocast. Gradients for allreduce (reduce-scatter) and parameters for allgather (only for ZeRO3) of lower_precision_safe_modules are also downcasted to this dtype.
• lower_precision_safe_modules: Downcast for allreduce (reduce-scatter) and allgather (ZeRO3) are applied only to modules specified in this list. (The precision for PyTorch operators in forward/backward follows torch.autocast's policy, not this list.) You can set names of classes with their packages. If you don't set this item, DeepSpeed uses the default list: [torch.nn.Linear, torch.nn.Conv1d, torch.nn.Conv2d, torch.nn.Conv3d].

Note that we only maintain FP32 parameters with this feature enabled. For consistency, you cannot enable fp16 or bf16 in DeepSpeed config.