Modify README for 3 pytorch examples

[yeandy]

Modify README for the 3 examples. They instruct users on how to set up data and models for the pipeline.

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[yeandy]

R: @AnandInguva @ryanthompson591 @tvalentyn @TheNeuralBit @rezarokni @pcoet

[ryanthompson591]

LGTM after Brians suggestions.

[codecov]

Codecov Report

Merging #21871 (05aae26) into master (18ab78c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21871      +/-   ##
==========================================
- Coverage   74.07%   74.05%   -0.02%     
==========================================
  Files         698      698              
  Lines       92574    92600      +26     
==========================================
+ Hits        68577    68578       +1     
- Misses      22742    22767      +25     
  Partials     1255     1255              

Flag	Coverage Δ
python	83.73% <ø> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
.../python/apache_beam/transforms/periodicsequence.py	96.77% <0.00%> (-1.59%)	⬇️
sdks/python/apache_beam/io/source_test_utils.py	88.01% <0.00%> (-1.39%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	90.06% <0.00%> (-1.33%)	⬇️
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
sdks/python/apache_beam/transforms/combiners.py	93.05% <0.00%> (-0.39%)	⬇️
sdks/python/apache_beam/pipeline.py	91.80% <0.00%> (ø)
sdks/python/apache_beam/utils/urns.py	88.70% <0.00%> (ø)
sdks/python/apache_beam/coders/coders.py	88.22% <0.00%> (ø)
sdks/python/apache_beam/runners/common.py	88.68% <0.00%> (ø)
... and 37 more

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