Increase the default max_length parameter when using TransfoXL & XLnet.
This is needed because the prepended PADDING_TEXT constant is already bigger than the default max_length parameter on the generate method, thus leading to no token being generated.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>
I don't think it is necessarly needed actually because the `max_length` is overwritten by the model's specific configs in this case. See XLNet config for example: https://s3.amazonaws.com/models.huggingface.co/bert/xlnet-base-cased-config.json. 

I would prefer to not set the `max_length` here, but let the user insert it if needed. After discussion with @thomwolf we decided to not hardcode parameter values in the pipelines.py file, but only set them via the `task_specific_params` in the configs.

I think the problem is that the task specific params are not overwritten correctly here because the task-name is not correct. 
It is important the the task-name `text-generation` is given to the pipeline, so that this line works as expected:
https://github.com/huggingface/transformers/blob/1ae132a07d7f294cf58cd50f7db8723d00e282de/src/transformers/pipelines.py#L400



<|||||>I tried to explain that here as well:https://github.com/huggingface/transformers/pull/5086#issuecomment-645847015 and think we should actually not pass the task name to the `__init__` function of the pipelines, but change the variable `task` to a static class variable @julien-c @LysandreJik @mfuntowicz @thomwolf <|||||>@patrickvonplaten Ok, thanks for the hints, I'll check the point you mentioned 👌 <|||||>Just a note that in the inference-api we *do* pass the correct "text-generation" task name, so there might be something else going on here.<|||||>Ok I'll check as well<|||||>I think I know what's going on - will do a PR that should fix it<|||||>This line in the inference api should not be called `causal-lm`, but `text-generation` IMO: https://github.com/huggingface/api-inference/blob/9cab899965d164f85c4961f0deafbc5034523e45/shared.py#L47 
This way the correct parameters would be loaded from the config.

But I would prefer to actually make the "task" name a static variable as is shown here:
https://github.com/huggingface/transformers/pull/5216

@julien-c @mfuntowicz <|||||>Oh yes my bad @patrickvonplaten, this was actually on a branch: https://github.com/huggingface/api-inference/pull/3/files