Improve documentation for Multi-weight support

[datumbox]

📚 The doc issue

Previously we have restructured the documentation for models (#5833, #5987, #5897 etc), so that we can add necessary information about how the models were trained and integrate with the new Multi-weight support API.

This ticket is a tracker for all remaining tasks related to model documentation + Multi-weight support API:

• Clean up the documentation pages Clean up model documentation #6003 Fix SSD documentation #6002
• Include information about the preprocessing transforms of models Add preprocessing information on the Weights documentation #6009
• Add examples on how to use the new Multi-weight model builders Add examples of Multi-weight support + model usage #6013
• Move from the old models.srt any useful info to the new one (TORCH_HOME + TorchHub info, train/eval modes etc) Expose get_weight to Torch Hub #6026 Add more info on new models.srt #6025
• Update galleries to use new idioms (for example fetching classes from meta-data and the text concerning preprocessing) Update galleries to use Multi-weight idioms #6030
• Add description to pre-trained weights that clarify their scope Document all pre-trained Classification weights #6036 Document all remaining pre-trained weights #6039

Suggest a potential alternative/fix

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