[Enhancement] Make build_xxx_layer allow accepting a class type

[HAOCHENYE]

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Motivation

Make build_xxx_layer accept cfg dict with a class type

In MMEngine, build_from_cfg allows accepting a read class type like this:

from mmengine.hooks import LoggerHook

cfg = dict(type=LoggerHook)
hook = build_from_cfg(cfg)

However, build_xxx_layer in MMCV does not allow accepting a class type:

from mmcv.cnn import build_conv_layer
from torch.nn import Conv2d

cfg = dict(type=Conv2d, ...)
layer = build_conv_layer(cfg)  # will raise an error

This PR makes build_xxx_layer can build layer from the class type.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with 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

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.