[New Model] Add MiniCPM3 support 
Author: aliyevaladddin
Pull request: yes

# 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

  - [x] 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).
  - [x] Did you read the [contributor guideline](https://github.com/huggingface/
  transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),                
        Pull Request section?
  - [x] Was this discussed/approved via a Github issue or the                   
  [forum](https://discuss.huggingface.co/)? Please add a link                   
        to it if that's the case. — #41115
  - [ ] Did you make sure to update the documentation with your changes?        
  - [ ] Did you write any new necessary tests?                                  
   
  ## Who can review?                                                            
                  
  @ArthurZucker @Cyrilvallez         

Comments:

  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


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


  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. 

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

run-slow: auto, minicpm3

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=45613&sha=889fab

Can we close this for #41116 then?