Remove prints from image dataset from directory

[EtiNL]

I added a verbose arg to index_directory function in utils/dataset_utils.py and to image_dataset_from_directory function in utils/image_dataset_utils.py to control/remove prints when using image_dataset_from_directory.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 75.51%. Comparing base (c8700f4) to head (0817213).
Report is 67 commits behind head on master.

Files	Patch %	Lines
keras/utils/dataset_utils.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19228      +/-   ##
==========================================
- Coverage   80.14%   75.51%   -4.63%     
==========================================
  Files         341      363      +22     
  Lines       36163    39232    +3069     
  Branches     7116     7586     +470     
==========================================
+ Hits        28982    29628     +646     
- Misses       5578     7968    +2390     
- Partials     1603     1636      +33     

Flag	Coverage Δ
keras	75.37% <75.00%> (-4.62%)	⬇️
keras-jax	59.74% <75.00%> (-3.32%)	⬇️
keras-numpy	54.15% <75.00%> (-2.93%)	⬇️
keras-tensorflow	60.89% <75.00%> (-3.77%)	⬇️
keras-torch	60.37% <75.00%> (-3.50%)	⬇️

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[fchollet]

LGTM, thanks