[Pallas:MGPU] Make the shapes from the attention example more interesting

This bumps up the number of heads and removes the batch_size=2 case: it's
very similar to batch_size=1 and doubles the script runtime. We also don't
do full autotuning by default since the largest size that works usually
performs the best.

PiperOrigin-RevId: 701976192

Author: apaszke

jax/experimental/pallas/ops/gpu/attention_mgpu.py

@@ -249,25 +249,26 @@ def attention_reference(q, k, v):
 
 
 def main(unused_argv):
-  num_q_heads = 1
-  num_kv_heads = 1
-  problem_it = itertools.product((1, 2), (4096, 32768,), (64, 128, 256,))
+  num_q_heads = 16
+  num_kv_heads = 16
+  problem_it = itertools.product((1,), (4096, 32768,), (64, 128, 256,))
   for batch_size, seq_len, head_dim in problem_it:
     q_seq_len = kv_seq_len = seq_len
     print(f"==== {batch_size=:<6} {kv_seq_len=:<6} {q_seq_len=:<6}"
           f"{num_q_heads=:<4} {head_dim=:<6} ====")
-    param_it = itertools.product((64,), (64, 128, 256))
-    best = None
     k1, k2, k3 = jax.random.split(jax.random.key(42), 3)
     q = jax.random.normal(k1, (batch_size, q_seq_len, num_q_heads, head_dim), jnp.float16)
     k = jax.random.normal(k2, (batch_size, kv_seq_len, num_kv_heads, head_dim), jnp.float16)
     v = jax.random.normal(k3, (batch_size, kv_seq_len, num_kv_heads, head_dim), jnp.float16)
-    for block_q, block_kv in param_it:
+    block_q = 64
+    best = None
+    for block_kv in (256, 128, 64):
       config = TuningConfig(block_q=block_q, block_kv=block_kv, max_concurrent_steps=2)
       try:
         out, runtime_ms = profiler.measure(functools.partial(attention, config=config), q, k, v)
-        out_ref = attention_reference(q, k, v)
-        np.testing.assert_allclose(out, out_ref, atol=2e-3, rtol=1e-3)
+        if seq_len < 32768:
+          out_ref = attention_reference(q, k, v)
+          np.testing.assert_allclose(out, out_ref, atol=2e-3, rtol=1e-3)
       except ValueError as e:
         if "exceeds available shared memory" in e.args[0]:
           continue
@@ -285,6 +286,7 @@ def main(unused_argv):
       )
       if best is None or runtime_us < best[0]:
         best = (runtime_us, achieved_tc_util)
+      break  # Remove this for full autotuning.
     if best is not None:
       print(f"Best: {best[0]:<7.1f}us = {best[1]:4.1f}% TC utilization")