[New Model] Add MiniCPM3 support

[aliyevaladddin]

What does this PR do?

Adds native support for the MiniCPM3 model architecture
(openbmb/MiniCPM3-4B).

MiniCPM3 uses Multi-head Latent Attention (MLA) from DeepSeek-V2 combined with
a standard dense MLP (no MoE), plus three scaling mechanisms:

• scale_emb — embedding scaling
• scale_depth / sqrt(num_hidden_layers) — residual connection scaling
• hidden_size / dim_model_base — logit scaling before the LM head

The implementation uses the modular model pattern, inheriting from Llama
(config, MLP, model structure) and DeepSeek-V2 (MLA attention, rotary
embeddings).

Fixes #41115

Code Agent Policy

• I confirm that this is not a pure code agent PR.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other
  checks if that's the case).
• Did you read the [contributor guideline](https://github.com/huggingface/
  transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
  Pull Request section?
• Was this discussed/approved via a Github issue or the
  forum? Please add a link
  to it if that's the case. — Add Model Architecture for MiniCPM3 #41115
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@ArthurZucker @Cyrilvallez

[aliyevaladddin]

Add MiniCPM3 model support with configuration and modeling classes

• Implement MiniCPM3 using modular pattern (inherits Llama + DeepSeek-V2 MLA
  attention)
• Add embedding scaling (scale_emb), residual scaling (scale_depth), and logit
  scaling (dim_model_base)
• Register model in AutoConfig, AutoModel, AutoModelForCausalLM,
  AutoModelForSequenceClassification
• Verified forward pass with small config

[Rocketknight1]

There's an existing PR at #41116, but it's quite stale at this point

[aliyevaladddin]

Thanks @Rocketknight1! Since #41116 has been stale since October 2025, I'd like to take over
and submit a fresh PR with MiniCPM3 support using the current modular pattern. I have a working
implementation with tests ready — will open a PR shortly.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: auto, minicpm3