feat(mlu): Support MLU Backend

[zgplvyou]

Support Cambricon MLU device for MMaction2.

Motivation

Support Cambricon MLU device for MMaction2.

Modification

mmaction/apis/train.py : train on the default device.
tools/test.py : test on the default device.
mmaction/utils/distribution_env.py : choose the default device; build ddp and dp model.

BC-breaking (Optional)

No BC-breaking

Use cases (Optional)

you can use the native script of gpu devices to train the model on mlu devices without any change.

[CLAassistant]

All committers have signed the CLA.

[kennymckormick]

@zgplvyou , please help fix the lint problem

[zgplvyou]

train on mlu：

[zgplvyou]

pytest tests/

we got the result of 1 failed, 213 passed, 5 skipped, 333 warnings. and on the master branch, we got the same result.

[zgplvyou]

code coverage:

we got the same percent(85%) code coverage as master. And we need run the tests on mlu device to cover the file of "mmaction/utils/distribution_env.py".

[HarryZJ]

pytest tests/ we got the result of 1 failed, 213 passed, 5 skipped, 333 warnings. and on the master branch, we got the same result.

@kennymckormick it seems like the test_rawframe_decode requires libturbojpeg library, but we didn't installed it in our system. That's how i guess.

[kennymckormick]

LGTM, is it OK to merge this PR now?

[HarryZJ]

LGTM

[OpenMMLab-Assistant-004]

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