Dead code removal in canonicalizer.

[Rishav1]

Implement something like Vulture's NodVisitor to detect and purge non-reachable dead code, unused imports and functions. It would prevent generation of unused DAGs in ONNX.

The current canonicalizer translates this code snippet below

def func():
    do_something1()
    return
    do_something2()  # Should be purged from AST as it will never be hit.

to following.

def func():
    returned_value = None
    returned1 = False
    do_something1()
    returned_value = None
    returned1 = True
    if not returned1:
        do_something2()  # This dead code is being processed in Elichika.
    return returned_value

If we implement a dead code removal canonicalizer, the above can be avoided.