constant tensors

[martinjrobins]

constant tensors that do not depend on input parameters should be able to be written directly to data as its being created.

perhaps Compiler::get_new_data() should write these tensors to the data before its returned. Will need to handle sensitivity data, which is just all zero

since we will pre-calculate constant tensors they can be removed from all other functions

[martinjrobins]

might need to also add this tensor storage to another data layout (e.g. constants) since we know they will not be involved in gradients so can mark as const

[martinjrobins]

actually, constant tensors can be written in the generated code as a global variable