Add: tensorflow example for image classification task guide
This PR addresses https://github.com/huggingface/transformers/issues/21037
It adds a Tensorflow example to the existing task guide on image classification. 

State of the PR: 
The example illustrates preprocessing in TF, training, and pushing to Hub. The code samples have been tested and they work/reproduce. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you all for the reviews! 
I have added the final part - data augmentation (kudos to @sayakpaul for helping me troubleshoot the issues I was having). The example is now complete. Let me know if it looks good enough to be merged :) 
cc @amyeroberts @sgugger 