[Unity] Remove unused local function definitions

[Lunderberg]

Prior to this commit, the relax.analysis.remove_all_unused utility only removed variable bindings that occur within a dataflow block. This satisfies that the condition that a variable can be removed if is unused and if its computation has no side effects. However, it doesn't include any values from a BindingBlock.

This commit updates the RemoveUnusedVars pass to also inspect BindingBlock instances for removeable variables. Currently, this is limited to unused local function definitions.

[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• cc @quic-sanirudh _{See #10317 for details}

_{Generated by tvm-bot}

[kparzysz-quic]

Shouldn't the two unuseds be removed? I think this whole algorithm (not your change specifically) is somewhat deficient if it doesn't keep track of newly-unused objects.

[Lunderberg]

For this specific change, because (to my understanding), the call_dps_packed doesn't guarantee that it has no side effects. Within a R.dataflow() environment, the expression may not have side effects, so it is safe to remove.

For newly-unused objects, I believe that is handled by the GetUnusedVars function, called during construction. It iteratively walks from the function outputs to the predecessors required to generate those outputs, finding the set of all variables required to generate the output. Anything not within that set is considered unused and can be removed, even it occurs within another expression (because that expression is also going to be removed).