feat: Add MPNet model family

[georgeguimaraes]

This PR adds support for the MPNet model family, enabling models like sentence-transformers/all-mpnet-base-v2 to be used in Bumblebee.

I noticed there was a previous attempt at this in #405 by @snewcomer, but it seems to have gone stale. I used the feedback from that PR review (thanks @jonatanklosko!) to create this implementation.

Changes

• Added Bumblebee.Text.Mpnet module with support for all standard architectures:
  • :base
  • :for_masked_language_modeling
  • :for_sequence_classification
  • :for_token_classification
  • :for_question_answering
  • :for_multiple_choice
• Registered MPNet models in Bumblebee.load_model/2
• Added :mpnet tokenizer type with the correct special tokens
• Added tests for all architectures

Notes

• MPNet does not use token_type_ids (unlike BERT)
• MPNet uses relative position bias shared across all encoder layers
• The HuggingFace layer names use shortened forms (q, k, v, o instead of query, key, value, output)

Closes #219

[Copilot]

Pull request overview

This PR adds comprehensive support for the MPNet model family to Bumblebee, enabling the use of models like sentence-transformers/all-mpnet-base-v2. The implementation follows the patterns established by similar encoder models (BERT, RoBERTa) in the codebase.

• Implements all standard MPNet architectures (base, masked LM, sequence/token classification, question answering, multiple choice)
• Adds proper tokenizer configuration with MPNet-specific special tokens
• Registers all model types in the main Bumblebee module
• Includes comprehensive test coverage for all architectures using tiny random models from HuggingFace

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lib/bumblebee/text/mpnet.ex	Core MPNet implementation with all architectures, configuration options, and HuggingFace parameter mappings
lib/bumblebee/text/pre_trained_tokenizer.ex	Adds MPNet tokenizer configuration with appropriate special tokens
lib/bumblebee.ex	Registers MPNet model classes and tokenizer type mapping
test/bumblebee/text/mpnet_test.exs	Comprehensive tests for all six MPNet architectures

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[jonatanklosko]

In all the tests we should assert against values from Python transformers, see #405 (comment). If it doesn't match, which seems to be the case here, it means there is still some implementation difference.