input: Write a function to create a virtual assistant using Python and Dialogflow to answer a basic set of questions. The function should prompt the user for input continuously, send the input to Dialogflow for processing, and print the assistant's response. If the user inputs 'quit', the function should terminate.
  import dialogflow_v2 as dialogflow
  
  def virtual_assistant(project_id):
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, 'session-id')
  
      while True:
          text = input('User: ')
          if text == 'quit':
              break
          text_input = dialogflow.types.TextInput(text=text, language_code='en-US')
          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)
  
  # Example usage
  assert virtual_assistant('project-id') == None
  # The function runs indefinitely until the user inputs 'quit'.
  # No explicit return value is needed for this function as it uses a loop to interact with the user.
output: def virtual_assistant(project_id):
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, 'session-id')
  
      while True:
          text = input('User: ')
          if text == 'quit':
              break
          text_input = dialogflow.types.TextInput(text=text, language_code='en-US')
          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)