[V1][Bugfix] Fix typo in MoE TPU checking

[ywang96]

Fixes the typo in #15834 that breaks two-node 4GPU test on CI since method object is always evaluated to True in Python when it's not being called.

Caught this bug from CI errors since we don't have TPU on CI but it's calling forward_tpu

[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]   File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/layers/fused_moe/layer.py", line 819, in forward
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]     return self.forward_impl(hidden_states, router_logits)
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]   File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/layers/fused_moe/layer.py", line 838, in forward_impl
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]     final_hidden_states = self.quant_method.apply(
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]   File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/layers/fused_moe/layer.py", line 161, in apply
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]     return self.forward(x=x,
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]   File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/custom_op.py", line 25, in forward
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]     return self._forward_method(*args, **kwargs)
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]   File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/layers/fused_moe/layer.py", line 295, in forward_tpu
[2025-04-01T08:01:38Z] ERROR 04-01 01:01:38 [core.py:376]     assert custom_routing_function is None

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[jeejeelee]

This also leads to a torch.compile error, see: #15871 (comment)

[ywang96]

I manually enabled 2 node 4 GPU test and confirmed this PR fixed it.