[Frontend][TFLite] PreLU alpha can be an expr (apache#11879)

* [Frontend][TFLite] PreLU alpha can be an expr

* [Frontend][TFLite] handle both cases of PreLU alpha param

python/tvm/relay/frontend/tflite.py

@@ -3010,11 +3010,14 @@ def convert_prelu(self, op):
 
         input_tensor = input_tensors[0]
         alpha_tensor = input_tensors[1]
-        alpha_tensor_type = alpha_tensor.tensor.Type()
-        alpha_tensor_type_str = self.get_tensor_type_str(alpha_tensor_type)
-        alpha_expr = self.exp_tab.new_const(
-            self.get_tensor_value(alpha_tensor), dtype=alpha_tensor_type_str
-        )
+        if self.has_expr(alpha_tensor.tensor_idx):
+            alpha_expr = self.get_expr(alpha_tensor.tensor_idx)
+        else:
+            alpha_tensor_type = alpha_tensor.tensor.Type()
+            alpha_tensor_type_str = self.get_tensor_type_str(alpha_tensor_type)
+            alpha_expr = self.exp_tab.new_const(
+                self.get_tensor_value(alpha_tensor), dtype=alpha_tensor_type_str
+            )
         in_expr = self.get_expr(input_tensor.tensor_idx)
         data_shape = to_int_list(self.get_tensor_shape(input_tensor))