[Fix] Add load_url to handle incompatibility of PyTorch versions

[zhouzaida]

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Motivation

Add load_url to handle incompatibility of PyTorch versions.

The 1.6 release of PyTorch switched torch.save to use a new zipfile-based file format. It will cause RuntimeError when a
checkpoint was saved in torch >= 1.6.0 but loaded in torch < 1.7.0. More
details at open-mmlab/mmpose#904

Modification

Add load_url function.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

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.

[codecov]

Codecov Report

Merging #1377 (0aad9c5) into master (8cac7c2) will decrease coverage by 0.00%.
The diff coverage is 57.14%.

❗ Current head 0aad9c5 differs from pull request most recent head 3167705. Consider uploading reports for the commit 3167705 to get more accurate results

@@            Coverage Diff             @@
##           master    #1377      +/-   ##
==========================================
- Coverage   68.59%   68.59%   -0.01%     
==========================================
  Files         164      164              
  Lines       10891    10896       +5     
  Branches     1991     1992       +1     
==========================================
+ Hits         7471     7474       +3     
- Misses       3030     3031       +1     
- Partials      390      391       +1     

Flag	Coverage Δ
unittests	68.59% <57.14%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/runner/checkpoint.py	74.17% <57.14%> (-0.24%)	⬇️

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[zhouzaida]

please @innerlee have a look

[ZwwWayne]

Can be merged after resolving conflicts