insert_write_back_for_buffers_pass should inject copy_ nodes at the earliest possible spot.

[JacobSzwejbka]

🐛 Describe the bug

Currently the pass injects all the 'copy_' nodes at the end of the graph. It should instead insert them immediately following the last usage of the thing being copied into the original mutable buffer. By waiting till the end we are arbitrarily extending the lifetime of those tensors potentially wasting a non trivial amount of space in the memory plan.

Versions

.