* metal: support left and circular padding in GGML_OP_PAD
Align Metal with CPU, CUDA and Vulkan: shift the source coordinates by
the left paddings, wrap them around with the same wrap_around when
circular, and read the source through nb00, which also fixes a right
padding of a permuted source. A test case covers it.
Drop the f32_4 kernel: its selection is disabled as slower, and it
fails two pad cases once enabled.
* metal: use a function constant for the circular pad variant
Address review from ggerganov: replace the bool template with FC_PAD,
as FC_upscale_aa does, so the pad kernel is compiled once and
specialized per pipeline.