Clip floating point constants to bf16 range to avoid inf conversion
Author: sangeethabal
Pull request: yes

When running HuggingFace BERT (any size) fine-tuning tutorial with transformers version >= 4.21.0 and using XLA_USE_BF16=1 or XLA_DOWNCAST_BF16=1, I see NaNs in the loss after the first step.

# What does this PR do?
This PR addresses the issue where the model code passes a value that is out of range for XLA_USE_BF16=1 or XLA_DOWNCAST_BF16=1, so the conversion would cast it to -inf.

The NaNs likely come from the transformers library change: https://github.com/huggingface/transformers/pull/17306 . This PR replaced many lines which used to be -float(inf) (or other small constants) with torch.finfo().min. For torch.float32 the min value is -3.4028234663852886e+38 which is smaller than the bfloat16 minimum of -3.3895313892515355e+38. So the problem is that torch.finfo(torch.float32).min = -3.4028234663852886e+38 gets converted to -inf. When the original encoder_extended_attention_mask is 1, then encoder_extended_attention_mask becomes (1.0 - 1.0 ) * -inf which becomes NaN (via IEEE rule Inf * 0.0 = NaN).

This PR ensures torch.finfo(torch.bfloat16).min = -3.3895313892515355e+38 and not -inf. Then the results would not have Nans.

The following lines checks for XLA_USE_BF16 or XLA_DOWNCAST_BF16 environment variable and sets the dtype accordingly:
```
if is_torch_tpu_available():
    if os.environ.get("XLA_USE_BF16") == 1:
         return torch.bfloat16
    if os.environ.get("XLA_DOWNCAST_BF16") == 1:
         if t.dtype == torch.float:
             return torch.bfloat16
         if t.dtype == torch.double:
             return torch.float32
 ```
Referencing related issues: https://github.com/aws-neuron/aws-neuron-sdk/issues/593 and   https://github.com/pytorch/xla/issues/4152 
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