Documentation for exp_mask

[murphp15]

I understand this is not really a bug but I have no idea how to figure out what the method is doing.
None of the tests hit this method so I can't really debug it.
Sorry for the stupid questions but I'm all out of ways of understanding what it is suppose to do.

[chrisc36]

This method fixes the numbers that are not in the mask to be a very negative number (instead of just zero, as is usually done in masking). This is done so that if we exponentiate the tensor, the masked elements will become zero, or at least very close to zero.

In other words it masks out the tensor in log-space.