[Fix] Import Error on StarkHead

[nijkah]

Currently, Transformer registry or class does not exist in mmdet==3.x. I think it should be modified as BaseModule instead of Transformer.

[CLAassistant]

All committers have signed the CLA.

[nijkah]

The latest codecov does not work with python==3.7.x. Should we limit the version of codecov?

ERROR: Ignored the following versions that require a different python version: 1.1.0 Requires-Python >=3.8; 1.1.1 Requires-Python >=3.8; 1.1.2 Requires-Python >=3.8; 1.1.3 Requires-Python >=3.8; 1.2.0 Requires-Python >=3.8; 1.2.0rc1 Requires-Python >=3.8; 1.2.1 Requires-Python >=3.8; 1.2.2 Requires-Python >=3.8
ERROR: Could not find a version that satisfies the requirement codecov (from versions: none)
ERROR: No matching distribution found for codecov