Cache derivative function for different input sizes

[dfdx]

Currently we generate derivative functions that are bound to input sizes. If input size changes, user needs to repeat differentiation process which may be costly. Rather than put it on the user, we can maintain a cache of functions, one per a set of input sizes, and provide the user with a "meta" function that adaptively selects needed derivative function.

We also need to keep track of buffer sizes in memory dict. The easiest way should be to replace @get_or_create with something like @get_or_alloc that also checks the size of the buffer.

This issue indirectly depends on GPU codegen.