[Gluon][Tutorial] Tweak GROUP_SIZE_N (#7448)

fp16+d128 was falling off at 32K+ N_CTX due to pid scheduling issues.
Tweak `GROUP_SIZE_N` to get better perf at longer contexts.

```
Attention Z=4 H=32 D=128 causal=False:
     N_CTX  triton-fp16
0   4096.0  1278.753121
1   8192.0  1270.658425
2  16384.0  1263.709358
3  32768.0  1241.498713
4  65536.0  1216.785238
Attention Z=4 H=32 D=128 causal=True:
     N_CTX  triton-fp16
0   4096.0  1008.655006
1   8192.0  1143.577101
2  16384.0  1136.524452
3  32768.0  1166.343494
4  65536.0  1068.798635
```

This doesn't seem to significantly affect fp8 or d64 perf.

Author: Mogball

python/tutorials/gluon/01-attention-forward.py

@@ -894,7 +894,7 @@ def attention_forward(q, k, v, causal, sm_scale):
     BLOCK_M = 256
     BLOCK_N = 128
     SPLIT_M = BLOCK_M // 2
-    GROUP_SIZE_N = 8
+    GROUP_SIZE_N = 4 if causal else 1
     NUM_SMS = torch.cuda.get_device_properties("cuda").multi_processor_count
 
     desc_q = make_tensor_desc(q, shape=[y_dim, HEAD_DIM_K], strides=[HEAD_DIM_K, 1], block_shape=[SPLIT_M, HEAD_DIM_K])