[Feature] support registering partial functions and more

[C1rN09]

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Motivation

This PR is converted to DRAFT because I think this may break the design of Registry and may mess up some concept. I will re-examine the necessity and rationality of this change.

Support registering partial functions in MMEngine, solve #594

This feature might be required by FSDP for auto_wrap_policies

Modification

modify Registry to accept all Callable, including classes, functions, partial functions and more

BC-breaking (Optional)

No

Use cases (Optional)

import functools
from mmengine.registry import Registry
FUNCS = Registry('funcs')
@FUNCS.register_module('resnet')
def build_resnet(layers=50, activation='relu'):
    pass
build_resnet18 = functools.partial(build_resnet, layers=18)
FUNCS.register_module('resnet18', False, build_resnet18)
model = FUNCS.build(dict(type='resnet18', activation='silu'))

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2df5bc1) 78.10% compared to head (6a3ee9c) 78.11%.

❗ Current head 6a3ee9c differs from pull request most recent head aae512e. Consider uploading reports for the commit aae512e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #595   +/-   ##
=======================================
  Coverage   78.10%   78.11%           
=======================================
  Files         126      126           
  Lines        9039     9042    +3     
  Branches     1800     1800           
=======================================
+ Hits         7060     7063    +3     
  Misses       1666     1666           
  Partials      313      313           

Flag	Coverage Δ
unittests	78.11% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmengine/registry/build_functions.py	80.61% <100.00%> (+0.40%)	⬆️
mmengine/registry/registry.py	95.45% <100.00%> (+0.02%)	⬆️

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