Add more ways to load image data for classification and detection

[ethanwharris]

What does this PR do?

Adds:

• ImageClassificationData.from_images
• ObjectDetectionData.from_numpy
• ObjectDetectionData.from_images
• ObjectDetectionData.from_tensors

Fixes #1371

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• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #1372 (dc16e80) into master (d751b83) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1372      +/-   ##
==========================================
+ Coverage   92.77%   92.81%   +0.04%     
==========================================
  Files         285      285              
  Lines       12685    12755      +70     
==========================================
+ Hits        11768    11839      +71     
+ Misses        917      916       -1     

Flag	Coverage Δ
unittests	92.81% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
flash/image/classification/data.py	98.64% <100.00%> (+0.05%)	⬆️
flash/image/classification/input.py	100.00% <100.00%> (ø)
flash/image/detection/data.py	100.00% <100.00%> (ø)
flash/image/detection/input.py	100.00% <100.00%> (ø)
flash/text/question_answering/model.py	93.19% <0.00%> (+0.68%)	⬆️

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