Use torch.nn.utils.clip_grad_norm_ and add clip_grad_by_value support for TPU

[carmocca]

What does this PR do?

See title

• EPSILON is removed. PyTorch does not have it as an argument. It should not be necessary as gradients are unscaled before clipping. I'm not sure why MixedPrecisionPlugin had EPSILON=1e-5 instead of 1e-6. Maybe somebody has more info about this.
• improves docs to clarify link to mixed precision.
• Adds support for clip_grad_value_ on TPU

Related question on torch-xla: pytorch/xla#2884

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[codecov]

Codecov Report

Merging #7025 (a0a6ad9) into master (9ba76ce) will decrease coverage by 0%.
The diff coverage is 67%.

@@          Coverage Diff           @@
##           master   #7025   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         200     200           
  Lines       12992   12970   -22     
======================================
- Hits        11918   11891   -27     
- Misses       1074    1079    +5     

[kaushikb11]

LGTM!

[awaelchli]

There was a PR that did the exact opposite: #963

[carmocca]

There was a PR that did the exact opposite: #963

We apply the torch-xla patch for TPU, no need to duplicate the code ourselves

Props for the ancient PR reference though

[tchaton]

LGTM !