[Refactor] refactor _prepare_pos_embed in ViT to fix bug in loading old checkpoint

[Ben-Louis]

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Motivation

Fix the bug in #1656

Modification

With the implementation in #1656, ckpt_pos_embed_shape will be converted to a tuple, and will cause a bug in the judgment of whether to remove the cls token:

if (not self.with_cls_token and ckpt_pos_embed_shape[1] == self.pos_embed.shape[1] + 1):
    ...

In this PR, the removal of the cls token is moved ahead of the process of resizing pos_embed. This implementation has undergone accuracy verification.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

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Additional details and impacted files

@@             Coverage Diff             @@
##             test-1.0    #1679   +/-   ##
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  Misses              ?     8214           
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Flag	Coverage Δ
unittests	65.50% <0.00%> (?)

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