[BUGFIX] Fix test_mla_backends.py. Scale MLA projection weights to prevent numerical instability (#32529)

Signed-off-by: Vadim Gimpelson <vadim.gimpelson@gmail.com>

tests/v1/attention/test_mla_backends.py

@@ -504,6 +504,14 @@ def test_backend_correctness(
     W_UV = torch.randn(
         kv_lora_rank, num_q_heads, v_head_dim, dtype=dtype, device=device
     )
+
+    # Scale weights to produce realistic magnitude outputs.
+    # Without scaling, projection output has std ~sqrt(kv_lora_rank) ≈ 22.6,
+    # causing extreme attention scores and numerical instability in LSE merging.
+    weight_scale = 1.0 / (kv_lora_rank**0.5)
+    W_UK = W_UK * weight_scale
+    W_UV = W_UV * weight_scale
+
     kv_b_proj_weight = torch.cat([W_UK, W_UV], dim=-1)
 
     for i, backend in enumerate(BACKENDS_TO_TEST):