GPU Memory Allocation Limits

There could be a command line option to set the max GPU memory allocation. By default tensorflow pre-allocates 100% of the GPU memory. If its not needed, then enabling the user to set the appropriate limit could help with GPU sharing. Example code for tensorflow is the following:

## extra imports to set GPU options
import tensorflow as tf
from keras import backend as k

###################################
# TensorFlow wizardry
config = tf.ConfigProto()

# Don't pre-allocate memory; allocate as-needed
config.gpu_options.allow_growth = True

# Only allow a total of half the GPU memory to be allocated
config.gpu_options.per_process_gpu_memory_fraction = 0.1

# Create a session with the above options specified.
k.tensorflow_backend.set_session(tf.Session(config=config))
###################################