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verl/models/README.md

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+# Models
+Common modelzoo such as huggingface/transformers stuggles when using Pytorch native model parallelism. Following the design principle of vLLM, we keep a simple, parallelizable, highly-optimized with packed inputs in verl. 
+## Adding a New Huggingface Model
+### Step 1: Copy the model file from HF to verl
+- Add a new file under verl/models/hf
+- Copy ONLY the model file from huggingface/transformers/models to verl/models/hf
+
+### Step 2: Modify the model file to use packed inputs
+- Remove all the code related to inference (kv cache)
+- Modify the inputs to include only
+    - input_ids (total_nnz,)
+    - cu_seqlens (total_nnz + 1,)
+    - max_seqlen_in_batch: int
+- Note that this requires using flash attention with causal mask.
+
+### Step 2.5: Add tests
+- Add a test to compare this version and the huggingface version
+- Following the infrastructure and add tests to tests/models/hf
+
+### Step 3: Add a function to apply tensor parallelism
+- Please follow
+    - https://pytorch.org/docs/stable/distributed.tensor.parallel.html
+    - https://pytorch.org/tutorials/intermediate/TP_tutorial.html
+- General comments
+    - Tensor Parallelism in native Pytorch is NOT auto-parallelism. The way it works is to specify how model parameters and input/output reshards using configs. These configs are then registered as hooks to perform input/output resharding before/after model forward.
+
+### Step 4: Add a function to apply data parallelism
+- Please use FSDP2 APIs
+- See demo here https://github.com/pytorch/torchtitan/blob/main/torchtitan/parallelisms/parallelize_llama.py#L413
+
+### Step 5: Add a function to apply pipeline parallelism
+- Comes in Pytorch 2.4
+- Currently only in alpha in nightly version
+- Check torchtitan for more details
+