Fix KerasImageFileEstimator for model tuning

[yogeshg]

• add test for KerasImageFileEstimator used with CrossValidator
• fix pickling issue
• use new fitMultiple api and ensure thread safety
• fix tests to reflect new api
• bugfix setDefault
• bugfix HasOutputMode
• remove _validateParams
• avoid testing KIFT functionality in KIFEst tests

[yogeshg]

• add test for KerasImageFileEstimator used with CrossValidator Build #561 should fail.

#561.1 failed

PicklingError: Can't pickle <type 'function'>: attribute lookup builtin.function failed

#561.2 failed #561.3 failed

AttributeError: 'list' object has no attribute 'transform'

[codecov-io]

Codecov Report

Merging #109 into master will increase coverage by 0.78%. The diff coverage is 89.79%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   82.66%   83.45%   +0.78%     
==========================================
  Files          34       34              
  Lines        1990     1994       +4     
  Branches       44       44              
==========================================
+ Hits         1645     1664      +19     
+ Misses        345      330      -15

Impacted Files	Coverage Δ
python/sparkdl/transformers/keras_image.py	100% <100%> (ø)	:arrow_up:
python/sparkdl/transformers/tf_image.py	94.06% <100%> (-0.05%)	:arrow_down:
python/sparkdl/param/image_params.py	77.5% <100%> (+1.82%)	:arrow_up:
...n/sparkdl/estimators/keras_image_file_estimator.py	85.71% <88.63%> (+11.35%)	:arrow_up:
src/main/scala/com/databricks/sparkdl/Models.scala	85% <0%> (ø)	:arrow_up:
python/sparkdl/param/shared_params.py	85.32% <0%> (+1.83%)	:arrow_up:

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[yogeshg]

• [x] avoid testing if transform produces the right result
• [x] validate the params of the KIFT are set correctly e.g.has correct input & output columns
• [x] ensure thread safety
• [ ] _validateParams should also validate that the other params are specified (?)
• [x] Remove _validateFitParams
• [x] why did we need to check in _validateFitParams that the value of the input col param is not the name of one of the user-specified params ?

[yogeshg]

@smurching thanks for the review and help offline!! I added the requested changes, and made a few comments to your questions.

[yogeshg]

• [x] keep the newlines between import groups ln:32
• [x] thread safety
• [x] raise issue for adding validations for the other params
• [x] put parens not (A or B in C)
• [x] "Input column must be defined or fine-tuned" or better

[yogeshg]

• [x] ensure parameters copied correctly