Switch to the docker image provided by PyTorch

[zhouzaida]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Refer to open-mmlab/mmdetection#5862

Switch to the docker image provided by PyTorch and remove some workflows that verify mmcv whether works well in the CPU version of PyTorch 1.3.0 and 1.4.0.

Modification

modify .github/workflow/build.yml

BC-breaking (Optional)

No

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[ZwwWayne]

may leave a comment to indicate why python -m is necessary

[zhouzaida]

may leave a comment to indicate why python -m is necessary

done