feat: implement Whisper forward() for fine-tuning support (fixes #2678)

- Implemented cross-entropy loss computation in WhisperWarp.forward()
- Added fine-tuning example script and documentation
- Users can now fine-tune any Whisper model on custom data via FunASR training framework

examples/industrial_data_pretraining/whisper/README.md

@@ -0,0 +1,65 @@
+# Whisper Fine-tuning with FunASR
+
+Fine-tune OpenAI Whisper models on your own data using FunASR's training framework.
+
+## Supported Models
+
+- whisper-tiny / whisper-tiny.en
+- whisper-base / whisper-base.en
+- whisper-small / whisper-small.en
+- whisper-medium / whisper-medium.en
+- whisper-large-v1 / whisper-large-v2 / whisper-large-v3 / whisper-large-v3-turbo
+
+## Data Preparation
+
+Prepare data in JSONL format:
+
+```json
+{"key": "utt001", "source": "/path/to/audio1.wav", "target": "the transcription text"}
+{"key": "utt002", "source": "/path/to/audio2.wav", "target": "another transcription"}
+```
+
+## Fine-tuning
+
+```bash
+bash finetune.sh
+```
+
+Or customize directly:
+
+```python
+from funasr import AutoModel
+
+model = AutoModel(model="Whisper-large-v3", model_conf={"hub": "openai"})
+
+# Training uses the forward() method which computes cross-entropy loss
+# on (mel-spectrogram, token_ids) pairs
+```
+
+## Key Parameters
+
+| Parameter | Default | Description |
+|-----------|---------|-------------|
+| model | Whisper-large-v3 | Model size |
+| lr | 1e-5 | Learning rate (lower for larger models) |
+| max_epoch | 10 | Training epochs |
+| batch_size | 4 | Per-GPU batch size |
+| warmup_steps | 500 | LR warmup |
+
+## Tips
+
+- For Chinese fine-tuning, use `whisper-large-v3` (best multilingual base)
+- Freeze encoder for faster training: add `++train_conf.freeze_param="model.encoder"`
+- Use smaller learning rates (1e-5 ~ 5e-6) to avoid catastrophic forgetting
+- Recommended: 100+ hours of target-domain audio for meaningful improvement
+
+## After Fine-tuning
+
+```python
+from funasr import AutoModel
+
+# Load fine-tuned model
+model = AutoModel(model="/path/to/exp/whisper_finetune")
+result = model.generate(input="test.wav")
+print(result[0]["text"])
+```

examples/industrial_data_pretraining/whisper/finetune.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# Whisper Fine-tuning with FunASR
+#
+# This script fine-tunes OpenAI Whisper models on custom data using FunASR's training framework.
+# Supports: whisper-tiny, whisper-base, whisper-small, whisper-medium, whisper-large-v3
+#
+# Data format: JSONL with "audio" and "text" fields
+# {"key": "utt1", "source": "/path/to/audio.wav", "target": "transcription text"}
+
+export CUDA_VISIBLE_DEVICES=0,1
+
+model_name="Whisper-large-v3"
+train_data="data/train.jsonl"
+val_data="data/val.jsonl"
+output_dir="exp/whisper_finetune"
+
+python -m funasr.bin.train \
+    ++model="${model_name}" \
+    ++model_conf.hub="openai" \
+    ++train_data_set_list="${train_data}" \
+    ++valid_data_set_list="${val_data}" \
+    ++dataset_conf.batch_size=4 \
+    ++dataset_conf.num_workers=4 \
+    ++train_conf.output_dir="${output_dir}" \
+    ++train_conf.max_epoch=10 \
+    ++train_conf.lr=1e-5 \
+    ++train_conf.warmup_steps=500 \
+    ++optim="adam" \
+    ++optim_conf.lr=1e-5 \
+    ++scheduler="warmuplr" \
+    ++scheduler_conf.warmup_steps=500

funasr/models/whisper/model.py

@@ -65,9 +65,48 @@ class WhisperWarp(nn.Module):
 
     def forward(
         self,
+        speech: torch.Tensor = None,
+        speech_lengths: torch.Tensor = None,
+        text: torch.Tensor = None,
+        text_lengths: torch.Tensor = None,
+        **kwargs,
     ):
-        """Forward pass for training."""
-        pass
+        """Forward pass for training. Computes cross-entropy loss.
+
+        Args:
+            speech: (B, T, D) mel-spectrogram features
+            speech_lengths: (B,) lengths of each audio
+            text: (B, U) token IDs (with SOT/EOT tokens)
+            text_lengths: (B,) lengths of each text sequence
+        Returns:
+            dict with "loss" and optionally "stats"
+        """
+        if speech is None or text is None:
+            raise ValueError("forward() requires speech and text for training")
+
+        # Encoder
+        audio_features = self.model.encoder(speech)
+
+        # Decoder: shift text right for teacher forcing
+        # text format: [SOT, lang, task, ..., tokens, EOT]
+        decoder_input = text[:, :-1]
+        decoder_target = text[:, 1:]
+
+        # Decoder forward
+        logits = self.model.decoder(decoder_input, audio_features)
+
+        # Cross-entropy loss (ignore padding, token_id = -1 or pad)
+        loss = F.cross_entropy(
+            logits.reshape(-1, logits.size(-1)),
+            decoder_target.reshape(-1),
+            ignore_index=-100,
+        )
+
+        stats = {
+            "loss": loss.detach().item(),
+            "batch_size": speech.size(0),
+        }
+        return {"loss": loss, "stats": stats}
 
     def inference(
         self,