removed type mismatch warning from gradient propagation algorithm.  It was not a good idea. Its ok for a gradient to have a different type from the original thing.

Summary of changes:
 - Added implementations of functools.partial, defaultdict, any, all
 - Re-wrote relative imports to be absolute imports
 - Re-wrote conditional expressions to normal if/else expressions

added optional argument (warn_type) to gradient.grad_sources_inputs to disable gradient type checking