[Feature] Support unroll with MMDDP in darts algorithm

[pprp]

Motivation

1. Support unroll method in the search phase of darts.
2. Support DDP training.
3. Add docstring for darts algorithm.

Results:

Docstring coverage:

TestCase coverage:

[codecov[bot]]

Codecov Report

Base: 82.86% // Head: 84.01% // Increases project coverage by +1.14% :tada:

Coverage data is based on head (3a1a0e7) compared to base (0409adc). Patch coverage: 97.22% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x     #210      +/-   ##
===========================================
+ Coverage    82.86%   84.01%   +1.14%     
===========================================
  Files          190      190              
  Lines         8336     8496     +160     
  Branches      1298     1327      +29     
===========================================
+ Hits          6908     7138     +230     
+ Misses        1168     1095      -73     
- Partials       260      263       +3     

Flag	Coverage Δ
unittests	84.01% <97.22%> (+1.14%)	:arrow_up:

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

Impacted Files	Coverage Δ
...or/models/architectures/heads/darts_subnet_head.py	35.71% <ø> (ø)
mmrazor/models/algorithms/nas/darts.py	96.45% <97.20%> (+73.05%)	:arrow_up:
...els/mutables/mutable_module/diff_mutable_module.py	91.90% <100.00%> (+0.57%)	:arrow_up:
mmrazor/structures/subnet/fix_subnet.py	92.30% <0.00%> (+11.53%)	:arrow_up:

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