Fix CI and attempt to improve flaky tests

[krshrimali]

What does this PR do?

This PR was an attempt to fix the CI, and it turned out to be successful. :) Here is the summary of what's done:

1. In the style_transfer.py script, the dataset we download contains an image with one number of channels (data/coco128/images/train2017/000000000086.jpg) - and pystiche raised an error while trying to normalize. It looks like the image wasn't converted to RGB through our loader, and that is fixed in this PR.
2. test_speed seems to be flaky for Windows/Mac, and we are trying to be softer in this PR. Let's investigate again if they appear to be flaky in the future.
3. Minor changes in the example scripts, like changing batch_size to match input samples, and reducing epochs to 1.

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

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[codecov]

Codecov Report

Merging #1394 (520fea6) into master (3e4c8bb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1394   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files         285      286    +1     
  Lines       12778    12835   +57     
=======================================
+ Hits        11873    11926   +53     
- Misses        905      909    +4     

Flag	Coverage Δ
unittests	92.91% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/utilities/loading.py	98.52% <100.00%> (+1.42%)	⬆️
flash/text/question_answering/model.py	93.87% <0.00%> (-0.69%)	⬇️
flash/core/utilities/embedder.py	91.48% <0.00%> (ø)
flash/core/model.py	91.42% <0.00%> (+0.23%)	⬆️

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[krshrimali]

Once and if all the tests pass, I'll update the CHANGELOG. I don't want to trigger the CI again when it's not worth it. :)

[ethanwharris]

Awesome! LGTM 😃