Fix dynamic pipeline remainder loop start

[xintin]

When max_iv < rounding_stride the pipeline conditional returns init values and the remainder loop must start from 0 instead of pipelined_iterations. Use sympy.Piecewise to pick the correct start.

• In schedule.py, build_guarded_pipeline_with_remainder generates this structure:

if (max_iv >= num_stages + unroll_factor - 1):    // guard
    result = pipeline(prologue + kernel + epilogue)
else:
    result = init_values

remainder_loop(start=pipelined_iterations, count=max_iv, init=result)

The remainder loop always starts at pipelined_iterations, regardless of whether the guard was true or false. When the guard is false (the pipeline didn't run), the start should be 0, not pipelined_iterations.

[xintin]

waiting for ci to pass. saw a couple flaky test failures in the earlier PR.
if it is not flaky, i will fix them before merging.