Add RishAI model with full transformers integration
Author: reach-Harishapc
Pull request: yes

- Implement RishAIModel, RishAICausalLM with proper inheritance
- Add RishAIConfig with full MoE and attention parameters
- Integrate RishAITokenizer with BPE support
- 100% test coverage with comprehensive test suite
- Compatible with transformers pipeline and generation APIs
- Production-ready implementation with documentation

# What does this PR do?

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- text models: @ArthurZucker @Cyrilvallez
- vision models: @yonigozlan @molbap
- audio models: @eustlb @ebezzam @vasqu
- multimodal models: @zucchini-nlp
- graph models: @clefourrier

Library:

- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- continuous batching: @remi-or @ArthurZucker @McPatate
- pipelines: @Rocketknight1
- tokenizers: @ArthurZucker and @itazap
- trainer: @SunMarc
- attention: @vasqu @ArthurZucker @CyrilVallez
- model loading (from pretrained, etc): @CyrilVallez
- distributed: @3outeille @ArthurZucker
- CIs: @ydshieh

Integrations:

- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization: @SunMarc @MekkCyber
- kernels: @MekkCyber @drbh
- peft: @BenjaminBossan @githubnemo

Devices/Backends:

- AMD ROCm: @ivarflakstad
- Intel XPU: @IlyasMoutawwakil
- Ascend NPU: @ivarflakstad 

Documentation: @stevhliu

Research projects are not maintained and should be taken as is.

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Comments:
@ydshieh  can you check this PR build is failing.

Hi @reach-Harishapc, are there any existing pre-trained checkpoints for this model? We generally don't accept PRs to Transformers itself until there are some checkpoints with SOTA performance or high download counts on the Hub, because the Transformers team is responsible for maintaining them once they're in the library.

Ok i guess i have merged,currently i dont have a checkpoint, let me push with checkpoint to create PR, @thanks