[diffusion] amd: fix SGLANG_DIFFUSION_ATTENTION_BACKEND env var for diffusion attention backend selection (#16325)

Co-authored-by: root <root@mi300x8-008.atl1.do.cpe.ice.amd.com>

python/sglang/multimodal_gen/runtime/platforms/rocm.py

@@ -11,7 +11,6 @@ from typing import Any
 
 import torch
 
-import sglang.multimodal_gen.envs as envs
 from sglang.multimodal_gen.runtime.platforms.interface import (
     AttentionBackendEnum,
     DeviceCapability,
@@ -93,11 +92,6 @@ class RocmPlatform(Platform):
         head_size: int,
         dtype: torch.dtype,
     ) -> str:
-        logger.info(
-            "Trying SGLANG_DIFFUSION_ATTENTION_BACKEND=%s",
-            envs.SGLANG_DIFFUSION_ATTENTION_BACKEND,
-        )
-
         if selected_backend == AttentionBackendEnum.TORCH_SDPA:
             logger.info("Using Torch SDPA backend.")
             return "sglang.multimodal_gen.runtime.layers.attention.backends.sdpa.SDPABackend"
@@ -108,13 +102,10 @@ class RocmPlatform(Platform):
         elif selected_backend == AttentionBackendEnum.AITER:
             if dtype not in (torch.float16, torch.bfloat16):
                 logger.warning(
-                    "AITer backend only supports fp16/bf16 inputs but got dtype=%s. "
-                    "Falling back to Torch SDPA backend.",
+                    "AITer backend works best with fp16/bf16 inputs but got dtype=%s. "
+                    "Proceeding with AITer anyway.",
                     dtype,
                 )
-                # TODO: need to compare triton with sdpa as an alternative backend
-                return "sglang.multimodal_gen.runtime.layers.attention.backends.sdpa.SDPABackend"
-
             logger.info("Using AITer backend on ROCm.")
             return "sglang.multimodal_gen.runtime.layers.attention.backends.aiter.AITerBackend"