Move getenv from kernel launch __call__ to __init__ (#3969)

Per comment
#3907 (comment).

`getenv` in each kernel launch would cost extra host time in the e2e
model run, this change move it to `__init__` function to make it only
gets called 1 time.

Author: leonling-ll

third_party/intel/backend/driver.py

@@ -707,11 +707,11 @@ def __init__(self, src, metadata):
         self.signature = {idx: value for idx, value in src.signature.items()}
         src = make_launcher(self.constants, self.signature)
         self.mod = compile_module_from_src(src, "__triton_launcher")
+        # Serialize KernelArguments for SPIR-V Runner
+        self.serialize_kernel_args = os.getenv('TRITON_XPU_DUMP_SPIRV_KERNEL_ARGS', None)
 
     def __call__(self, *args, **kwargs):
-        # Serialize KernelArguments for SPIR-V Runner
-        serialize_kernel_args = os.getenv('TRITON_XPU_DUMP_SPIRV_KERNEL_ARGS', None)
-        if serialize_kernel_args:
+        if self.serialize_kernel_args:
             serialize_args(args, self.constants, self.signature)
         self.mod.launch(args)