[Enhance] Add ConvModule.turn_on_fast_conv_bn_eval to reduce repetitive code and dynamically bind conv during forward

[youkaichao]

Motivation

Reduce repetitive code for the fast_conv_bn_eval feature introduced in #2807 .

Modification

Create a simple turn_on_fast_conv_bn_eval function, and call this function in __init__ and create_from_conv_bn. In the future, mmengine might also require this function. See open-mmlab/mmengine#1202 for details.

This PR also subsumes the #2836 PR to dynamically bind conv during forward. Test examples added for this case.

BC-breaking (Optional)

It does not break backward-compatibility, as it just introduces a new function.

Use cases (Optional)

An existing ConvModule object can call turn_on_fast_conv_bn_eval to turn on the fast_conv_bn_eval feature.

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.