[Fix] fix SiLU activation

[LeoXing1996]

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Motivation

MMCV v 1.6.2 supportSiLU activation function and registers it to ACTIVATION_LAYERS. This conflict with MMGen's SiLU implementation. (#444)

Modification

Only register SiLU when MMCV version is lower than 1.6.2

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)

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Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
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• 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:

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