feat: more flexible model name by converting to lowercase

[ZiniuYu]

This PR converts the model name to lowercase. This allows more flexibility when inputting the model name since we get a lot longer model names and cause many typing errors wrt cases.

[github-actions[bot]]

:memo: Docs are deployed on https://ft-lower-case--jina-docs.netlify.app :tada:

[codecov[bot]]

Codecov Report

Merging #830 (6e91fe8) into main (2ba8a4f) will decrease coverage by 0.24%. The diff coverage is 96.15%.

@@            Coverage Diff             @@
##             main     #830      +/-   ##
==========================================
- Coverage   84.38%   84.13%   -0.25%     
==========================================
  Files          21       21              
  Lines        1575     1576       +1     
==========================================
- Hits         1329     1326       -3     
- Misses        246      250       +4     

Flag	Coverage Δ
cas	84.13% <96.15%> (-0.25%)	:arrow_down:

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

Impacted Files	Coverage Δ
server/clip_server/model/pretrained_models.py	98.41% <ø> (ø)
server/clip_server/model/openclip_model.py	89.65% <85.71%> (ø)
server/clip_server/executors/clip_onnx.py	84.72% <100.00%> (ø)
server/clip_server/executors/clip_tensorrt.py	93.10% <100.00%> (ø)
server/clip_server/executors/clip_torch.py	83.33% <100.00%> (ø)
server/clip_server/model/clip_model.py	91.66% <100.00%> (ø)
server/clip_server/model/clip_onnx.py	78.18% <100.00%> (ø)
server/clip_server/model/clip_trt.py	85.71% <100.00%> (ø)
server/clip_server/model/mclip_model.py	83.78% <100.00%> (+0.45%)	:arrow_up:
server/clip_server/model/tokenization.py	75.75% <0.00%> (-12.13%)	:arrow_down:

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[numb3r3]

I want to hold this PR. Because I cannot see the big benefits of this PR.