[TIR] Add preserve-unit-iters

[junrushao]

Follow-up of #11578, which enforces structural stability of TIR by
avoiding over-simplification in affine analysis. On the other hand, it
is possible that over-simplification could be desirable behavior.
Therefore, following the precedent of preserve-unit-loops in
Compute-At, this PR introduces preserve-unit-iters in block binding
for cases where users don't need structural stability (which is
admittedly rare).

This PR does not affect any existing functionalities.

Example:

# The original TIR
for i in T.serial(2):
    with T.block("C"):
        k = T.axis.reduce(2, i)

# Our operation is:
Split(i, [1, 2], preserve-unit-iters=True/False)

# preserve-unit-iters = True
for i_0, i_1 in T.grid(1, 2):
    with T.block("C"):
        k = T.axis.reduce(2, i_0 * 2 + i_1)

# preserve-unit-iters = False
for i_0, i_1 in T.grid(1, 2):
    with T.block("C"):
        k = T.axis.reduce(2, i_1)

[junrushao]

Per discussion with @spectrometerHBH. CC: @wrongtest @Hzfengsy @jinhongyii would you guys like to review this PR? Thanks a lot!

[Hzfengsy]

Can you add unittests for it? Then we can merge it quickly

[junrushao]

Per discussion with @Hzfengsy: we are going to use True as default given it's not gonna to affect existing functionalities. I'm going to update the PR later this week :-)

[junrushao]

Script to upgrade tuning record database:

import argparse
import json


def _parse_args():
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "--json-tuning-record",
        type=str,
        help="path to the json tuning record file",
        required=True,
    )
    parser.add_argument(
        "--updated-json-tuning-record",
        type=str,
        help="path to the new json tuning record file after upgrade",
        required=True,
    )
    return parser.parse_args()


ARGS = _parse_args()


def main():
    with open(ARGS.json_tuning_record, "r", encoding="utf-8") as i_f:
        lines = [json.loads(line) for line in i_f]
    for line in lines:
        for inst in line[1][0][0]:
            if inst[0] in ["Split", "Fuse"]:
                inst[2] = [1]
    with open(ARGS.updated_json_tuning_record, "w", encoding="utf-8") as o_f:
        for line in lines:
            o_f.write(json.dumps(line) + "\n")


if __name__ == "__main__":
    main()