metal : tighten input-position loop in kernel_conv_transpose_1d (ggml/1477)

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.

Author: CrispStrobe

ggml/src/ggml-metal/ggml-metal.metal

@@ -4881,15 +4881,32 @@ kernel void kernel_conv_transpose_1d(
         uint3   tgpig[[threadgroup_position_in_grid]],
         uint3   tgpg[[threadgroups_per_grid]]) {
 
-    float v = 0.0f;
+    // For 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).
+    const int32_t j  = tgpig[0];
+    const int32_t s0 = args.s0;
+    const int32_t K  = args.K;
+    const int32_t IL = args.IL;
+
+    int32_t i_min;
+    {
+        int32_t a = j - K + 1;
+        i_min = a <= 0 ? 0 : (a + s0 - 1) / s0; // ceil(a/s0) for a>0
+    }
+    int32_t i_max = j / s0;
+    if (i_max > IL - 1) i_max = IL - 1;
 
-    for (int64_t c = 0; c < args.IC; c++) {
-        const int32_t kernel_offset = c * tgpg[1] * args.K + args.K * tgpig[1];
-        const int32_t input_offset = c * args.IL;
+    float v = 0.0f;
+    if (i_min <= i_max) {
+        for (int64_t c = 0; c < args.IC; c++) {
+            const int32_t kernel_offset = c * tgpg[1] * K + K * tgpig[1];
+            const int32_t input_offset  = c * IL;
 
-        for (int64_t i = 0; i < args.IL; i++) {
-            if (tgpig[0] >= i * args.s0 && tgpig[0] < i * args.s0 + args.K) {
-                v += src0[kernel_offset + tgpig[0] - i * args.s0] * src1[input_offset + i];
+            for (int32_t i = i_min; i <= i_max; i++) {
+                v += float(src0[kernel_offset + j - i * s0]) * src1[input_offset + i];
             }
         }
     }