modifications for custom grayscale dataset (input format .hdf5)

[divya-nk]

myds_retrain.py:

• works with input format .hdf5 for a custom dataset
• modified code to work for HD grayscale image
• modified to work for more than one bounding box in an image.
• train-validation split implemented in this code

voc_conversion_scripts/convert_to_hdf5.py

• converts all images in the custom dataset into a single .hdf5 format file
• modified bounding box units to host coordiates of HD images