[Fix] Check the version of torchvision in __init__ of DCN

[mengpenghui]

Motivation

When performing the MLU-based PyTorch1.6 test, because PyTorch1.6-mlu does not support DCN,
the local test case will report an error.

Modification

mmcv/ops/deform_conv.py：
-Put the assert of torchvision version in the function
-Simplified weight syntax
mmcv/ops/modulated_deform_conv.py：
-Put the assert of torchvision version in the function

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.