For a given output position j on the time axis, only input positions
i such that i*s0 <= j < i*s0 + K contribute -- i.e.
i in [ceil((j - K + 1)/s0), floor(j/s0)] intersected with [0, IL-1].
That's at most ceil(K/s0) values (typically 2 for stride==K/2
transposed convs).
The current kernel iterates the full IL range and filters with an
`if`, amplifying per-thread work by IL/ceil(K/s0) (~160x for IL=320,
K=10, s0=5 -- a representative codec-decoder shape). On Apple M1
the wasted work trips the macOS GPU watchdog
(kIOGPUCommandBufferCallbackErrorImpactingInteractivity) on long
graphs.
Compute i_min, i_max analytically before the inner loop and iterate
only [i_min, i_max]. Output is bit-identical (same multiplies and
adds in the same order); loop bound shrinks by IL/ceil(K/s0).
Tested on M1 with a downstream consumer running a TTS codec at full
T_codec; end-to-end codec decode ~3-4x faster, zero watchdog hits
across long synthesis runs vs ~30% pre-patch.
`im2col_cuda` and `im2col_3d_cuda` both dispatch with
`block_nums.y = OW`. CUDA caps grid Y at 65535. Conv1d encoders on
raw 16 kHz audio with T > 65535 (~ 4 s) trip the limit -- e.g. SEANet
at 11 s lands at OW = 176000 -- and the launch returns
`invalid configuration argument`.
Clamp `block_nums.y` to `MIN(OW, MAX_GRIDDIM_Y)` and loop inside the
kernel with stride `MAX_GRIDDIM_Y`. Same in-kernel stride pattern
already used for the z axis (`MAX_GRIDDIM_Z`). Both 2D `im2col_kernel`
and 3D `im2col_3d_kernel` need the same fix. Bit-identical for
OW <= 65535 (single iteration of the new outer loop).
Tested on T4 / Jetson Orin with a SEANet encoder running on 11 s /
16 kHz audio (im2col reaching OW ~ 176000); pre-fix launch returns
`invalid configuration argument`, post-fix runs to completion.
Existing test-backend-ops im2col cases unchanged.
