[FIX] Fix bug loading IN1k dataset

[Ezra-Yu]

Motivation

if there is 'meta/train.txt' in the imagenet dataset folder.

the ann_file would be data_root/data_root/meta/train.txt

Modification

Please briefly describe what modification is made in this PR.

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: 44.26% and project coverage change: -1.17 ⚠️

Comparison is base (f9dcae2) 68.16% compared to head (1391aeb) 66.99%.

❗ Current head 1391aeb differs from pull request most recent head 2e29f78. Consider uploading reports for the commit 2e29f78 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1641      +/-   ##
==========================================
- Coverage   68.16%   66.99%   -1.17%     
==========================================
  Files         295      309      +14     
  Lines       23372    24610    +1238     
  Branches     3713     3904     +191     
==========================================
+ Hits        15932    16488     +556     
- Misses       6880     7519     +639     
- Partials      560      603      +43     

Flag	Coverage Δ
unittests	66.99% <44.26%> (-1.17%)	⬇️

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

Impacted Files	Coverage Δ
mmpretrain/apis/image_retrieval.py	21.42% <ø> (ø)
mmpretrain/datasets/__init__.py	66.66% <0.00%> (-7.62%)	⬇️
mmpretrain/datasets/gqa_dataset.py	0.00% <0.00%> (ø)
mmpretrain/datasets/nocaps.py	0.00% <0.00%> (ø)
mmpretrain/datasets/ocr_vqa.py	0.00% <0.00%> (ø)
mmpretrain/datasets/scienceqa.py	0.00% <0.00%> (ø)
mmpretrain/datasets/textvqa.py	0.00% <0.00%> (ø)
mmpretrain/models/classifiers/image.py	89.83% <0.00%> (-3.16%)	⬇️
mmpretrain/models/multimodal/__init__.py	40.00% <0.00%> (-4.45%)	⬇️
...retrain/models/multimodal/chinese_clip/__init__.py	0.00% <0.00%> (ø)
... and 30 more

... and 1 file with indirect coverage changes

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