[TPU] Add Load-time W8A16 quantization for TPU Backend (#7005)

vllm/config.py

@@ -244,6 +244,7 @@ class ModelConfig:
             "fp8", "marlin", "gptq_marlin_24", "gptq_marlin", "awq_marlin",
             "fbgemm_fp8", "compressed_tensors", "compressed-tensors"
         ]
+        tpu_supported_quantization = ["tpu_int8"]
         if self.quantization is not None:
             self.quantization = self.quantization.lower()
 
@@ -282,6 +283,11 @@ class ModelConfig:
                 raise ValueError(
                     f"{self.quantization} quantization is currently not "
                     f"supported in ROCm.")
+            if is_tpu(
+            ) and self.quantization not in tpu_supported_quantization:
+                raise ValueError(
+                    f"{self.quantization} quantization is currently not "
+                    f"supported in TPU Backend.")
             if self.quantization not in optimized_quantization_methods:
                 logger.warning(
                     "%s quantization is not fully "