[Enhancement] Speed import mmcv

[zhouzaida]

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Motivation

Speed import mmcv

Modification

• Refactor the checking logic whether ROCM_HOME can be obtained.
• Remove CUDA_HOME from mmcv/utils/parrots_wrapper.py

BC-breaking (Optional)

I am not sure whether the downstream codebases directly use the CUDA_HOME which is removed from mmcv/utils/parrots_wrapper.py in this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1249 (859fb4c) into master (04daea4) will decrease coverage by 0.08%.
The diff coverage is 8.33%.

❗ Current head 859fb4c differs from pull request most recent head 5003ff1. Consider uploading reports for the commit 5003ff1 to get more accurate results

@@            Coverage Diff             @@
##           master    #1249      +/-   ##
==========================================
- Coverage   68.27%   68.18%   -0.09%     
==========================================
  Files         160      160              
  Lines       10599    10603       +4     
  Branches     1937     1937              
==========================================
- Hits         7236     7230       -6     
- Misses       2979     2992      +13     
+ Partials      384      381       -3     

Flag	Coverage Δ
unittests	68.18% <8.33%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
mmcv/utils/__init__.py	88.88% <ø> (ø)
mmcv/utils/env.py	55.55% <0.00%> (-1.05%)	⬇️
mmcv/utils/parrots_wrapper.py	47.76% <10.00%> (-11.62%)	⬇️

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