[DOT NOT MERGE] Tokenizers Shape Polymorphism - Introduce pad_to_next_multiple_of parameters
Needs new release of tokenizers (cc @n1t0) 
That's a really cool feature. Looking forward to it!<|||||>Is the bucketization size not going to be too linear? Shouldn't we rather do `pad_to_next_power_of_two` or similar?<|||||>@julien-c That will be linear yet, by using a `power_of` growth we might rapidly increase the number of padding tokens to add and then fall into the opposite situation where most of the computation will be wasted on padding tokens.