SpanFinder: set default max_length to 25

[adrianeboyd]

Description

When the default max_length is not set and there are longer training documents, it can be difficult to train and evaluate the span finder due to memory limits and the time it takes to evaluate a huge number of predicted spans.

Types of change

Enhancement.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.