Support variables in base files for configs

[innerlee]

You can reference variables in base files now. See the test file for examples.

[codecov]

Codecov Report

Merging #1083 (5a41a39) into master (eb08835) will increase coverage by 0.10%.
The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master    #1083      +/-   ##
==========================================
+ Coverage   67.74%   67.84%   +0.10%     
==========================================
  Files         159      159              
  Lines       10337    10376      +39     
  Branches     1873     1885      +12     
==========================================
+ Hits         7003     7040      +37     
- Misses       2966     2967       +1     
- Partials      368      369       +1     

Flag	Coverage Δ
unittests	67.84% <94.87%> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
mmcv/utils/config.py	90.38% <94.87%> (+0.50%)	⬆️

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[innerlee]

Open for discussion: should we use {{ base.xxx }} or {{ _base_.xxx }}

Benefit of base: clean, short.
Benefit of _base_: can pass flake8.

[ZwwWayne]

Open for discussion: should we use {{ base.xxx }} or {{ _base_.xxx }}

Benefit of base: clean, short.
Benefit of _base: can pass flake8.

suggest using _base_ to keep consistency with that in the file header.

[innerlee]

@ZwwWayne pls check the comment to see if the _ was used as emph in markdown

[innerlee]

Another "pro" for base is that we can extend the gramma to "env"

[ZwwWayne]

@ZwwWayne pls check the comment to see if the _ was used as emph in markdown

should be _base_

[ZwwWayne]

Need to resolve confilcts.

[innerlee]

ping @ZwwWayne ~~