Fix #33773: Replace unconditional pandas import with PlaceholderModule (#39990)

Signed-off-by: Netanel Haber <58652339+netanel-haber@users.noreply.github.com>

vllm/_aiter_ops.py

@@ -3,18 +3,23 @@
 import functools
 from collections.abc import Callable
 
-import pandas as pd
 import torch
 from torch._ops import OpOverload
 
 import vllm.envs as envs
 from vllm.platforms import current_platform
+from vllm.utils.import_utils import PlaceholderModule
 from vllm.utils.torch_utils import direct_register_custom_op
 from vllm.v1.attention.ops.rocm_aiter_mla_sparse import (
     rocm_aiter_sparse_attn_indexer,
     rocm_aiter_sparse_attn_indexer_fake,
 )
 
+try:
+    import pandas as pd
+except ImportError:
+    pd = PlaceholderModule("pandas")
+
 # fp8_dtype is not cached.
 # on ROCm the fp8_dtype always calls is_fp8_fnuz
 # which is a host op, so we cache it once here.