"False".
     * average_attn_weights: If true, indicates that the returned
       "attn_weights" should be averaged across heads. Otherwise,
       "attn_weights" are provided separately per head. Note that
       this flag only has an effect when "need_weights=True.".
       Default: True (i.e. average weights across heads)

     * Outputs:

     * attn_output: (L, N, E) where L is the target sequence
       length, N is the batch size, E is the embedding dimension.
       (N, L, E) if "batch_first" is "True".

     * attn_output_weights: If "average_attn_weights=True",
       returns attention weights averaged across heads of shape
       (N, L, S), where N is the batch size, L is the target
       sequence length, S is the source sequence length. If
       "average_attn_weights=False", returns attention weights per
       head of shape (N, num_heads, L, S).
