input: Write a function to create a virtual assistant using Python and Dialogflow to answer a basic set of questions. The function should continuously prompt the user for input, send the input to Dialogflow for processing, and print the assistant's response. If the Dialogflow project ID and session ID are not provided, the function should raise a ValueError.
  assert create_virtual_assistant('project-id', 'session-id')
                  # Output: Continuous loop prompting for user input and printing assistant responses
  assert create_virtual_assistant('', 'session-id')==ValueError
                  # Output: ValueError
  assert create_virtual_assistant('project-id', '')==ValueError
                  # Output: 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)