Enable TruncationStrategy override for pipelines
Author: Narsil
Pull request: yes

# What does this PR do?

Right now truncation argument for tokenizers was not overridable, which leads to poor UX on some pipelines, most notably Summarization. Summaries trigger an error on text that end up with too many tokens for the underlying model.

Current strategy is just to enable the argument to be overrided as truncating by default is not necessarily good either.
More complex strategies are required to "solve" the problem (chunk original text into chunk of ~max_length, drop if some chunk is small enough <0.1 max_length?, then concatenate result summaries ?).

The current PR is a small step in that direction.
There should not be any backward incompatibilities with current changes.



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