[Feature] Add is_tracing to wrap torch.jit.is_tracing in different versions.

[mzr1996]

Motivation

Head needs different behaviors in test and export, so we need to use torch.jit.is_tracing to judge different situations.

But torch.jit.is_tracing has a bug in PyTorch 1.6.0, so we need a wrapper to support trace judgement.

Modification

Add a helper function in mmcv/utils, and add unit tests.

BC-breaking (Optional)

No

Use cases (Optional)

import torch
from mmcv.utils import is_tracing

def foo(x):
    if is_tracing():
        return x
    else:
        return x.tolist()

>>> x = torch.ones(3)
>>> foo(x)
[1.0, 1.0, 1.0]
>>> traced_foo = torch.jit.trace(foo, (torch.rand(1), ))
>>> traced_foo(x)
tensor([1., 1., 1.])

[codecov]

Codecov Report

Merging #1187 (6a43133) into master (4a9f834) will decrease coverage by 0.02%.
The diff coverage is 59.09%.

❗ Current head 6a43133 differs from pull request most recent head cb5ebd8. Consider uploading reports for the commit cb5ebd8 to get more accurate results

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
- Coverage   67.99%   67.96%   -0.03%     
==========================================
  Files         159      160       +1     
  Lines       10426    10446      +20     
  Branches     1897     1901       +4     
==========================================
+ Hits         7089     7100      +11     
- Misses       2968     2973       +5     
- Partials      369      373       +4     

Flag	Coverage Δ
unittests	67.96% <59.09%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/utils/trace.py	54.54% <54.54%> (ø)
mmcv/utils/parrots_wrapper.py	59.37% <55.55%> (-1.34%)	⬇️
mmcv/utils/__init__.py	88.88% <100.00%> (+0.65%)	⬆️
mmcv/version.py	92.30% <100.00%> (ø)

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