Changing XLNet default from not using memories to 512 context size following paper
In #8317, we found out that calling `XLNetLMHeadModel.from_pretrained(mem_len=384)` still produced the FutureWarning that announces that the default configuration will change in 3.5.0, as the config first gets initialized with `mem_len=0` then `mem_len` gets changed. This PR moves the warning to the `forward` pass in the model to avoid this.
Ha yeah I saw your message on Slack just afterwards, I'll just change the default :)<|||||>It's changed !<|||||>Merging now so that it's in v3.5.0!