[Refactor] BEiT refactor

[fanqiNO1]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

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:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage: 37.88% and project coverage change: -2.75 ⚠️

Comparison is base (f9dcae2) 68.16% compared to head (7043663) 65.41%.

❗ Current head 7043663 differs from pull request most recent head 2fb930b. Consider uploading reports for the commit 2fb930b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1705      +/-   ##
==========================================
- Coverage   68.16%   65.41%   -2.75%     
==========================================
  Files         295      330      +35     
  Lines       23372    25736    +2364     
  Branches     3713     4105     +392     
==========================================
+ Hits        15932    16836     +904     
- Misses       6880     8282    +1402     
- Partials      560      618      +58     

Flag	Coverage Δ
unittests	65.41% <37.88%> (-2.75%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmpretrain/apis/feature_extractor.py	37.50% <0.00%> (ø)
mmpretrain/apis/image_caption.py	30.64% <0.00%> (ø)
mmpretrain/apis/image_retrieval.py	21.42% <0.00%> (ø)
mmpretrain/apis/visual_grounding.py	27.53% <0.00%> (ø)
mmpretrain/apis/visual_question_answering.py	25.67% <0.00%> (ø)
mmpretrain/datasets/__init__.py	60.46% <0.00%> (-13.83%)	⬇️
mmpretrain/datasets/flickr30k_caption.py	0.00% <0.00%> (ø)
mmpretrain/datasets/flickr30k_retrieval.py	0.00% <0.00%> (ø)
mmpretrain/datasets/gqa_dataset.py	0.00% <0.00%> (ø)
mmpretrain/datasets/nocaps.py	0.00% <0.00%> (ø)
... and 68 more

... and 3 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.