input: Write a function to create a virtual assistant using Python and Dialogflow that continuously takes user input and returns the assistant's response. The function should use the Dialogflow API to detect user intent and provide the fulfillment text as the response. If the project ID or session ID is not provided, the function should raise an error.
  assert create_virtual_assistant('project-id', 'session-id')==None
  assert create_virtual_assistant('project-id', None)==ValueError
  assert create_virtual_assistant(None, 'session-id')==ValueError
  assert create_virtual_assistant(None, None)==ValueError
output: def create_virtual_assistant(project_id, session_id):
      if not project_id or not session_id:
          raise ValueError
      import dialogflow_v2 as dialogflow
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, session_id)
      while True:
          user_input = input("You: ")
          text_input = dialogflow.types.TextInput(text=user_input, language_code='en')
          query_input = dialogflow.types.QueryInput(text=text_input)
          response = session_client.detect_intent(session=session, query_input=query_input)
          print("Assistant:", response.query_result.fulfillment_text)