Sync MOE layer input quantizer only (#903)

## What does this PR do?

**Type of change:** Bug fix

**Overview:** in MOE layer we currently sync both the weight and input
quantizers so that all experts have the same weight amaxes and
activation amaxes.

VLLM/TRTLLM actually support non-uniform weight amaxes in MOE so we only
need to sync the activation amaxes.

## Usage
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```python
# Add a code snippet demonstrating how to use this
```

## Testing
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- **Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/Model-Optimizer/blob/main/CONTRIBUTING.md)**
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- **Is this change backward compatible?**: Yes/No <!--- If No, explain
why. -->
- **Did you write any new necessary tests?**: Yes/No
- **Did you add or update any necessary documentation?**: Yes/No
- **Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?**:
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## Additional Information
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved input quantizer synchronization for Mixture of Experts models
to ensure correct amax value handling across local experts.

* **Documentation**
  * Fixed typos and clarified wording in quantization documentation.

* **Tests**
* Added test coverage for Mixture of Experts quantizer synchronization
functionality.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jennifer Chen <jennifchen@nvidia.com>

Author: jenchen13

modelopt/torch/quantization/model_calib.py

@@ -96,13 +96,18 @@ def _check_moe_calibration_complete(quantizer, parallel_state):
 
 
 @torch.no_grad()
-def max_calibrate(model: nn.Module, forward_loop: ForwardLoop | None = None, distributed_sync=True):
+def max_calibrate(
+    model: nn.Module,
+    forward_loop: ForwardLoop | None = None,
+    distributed_sync=True,
+):
     """Calibrate the model using max.
 
     Args:
         model: Model to be calibrated.
         forward_loop: A callable which takes the model as argument and
             forwards calibration data through the model.
+        distributed_sync: Whether to sync input_quantizer amax across distributed processes.
 
     See :class:`MaxCalibConfig <modelopt.torch.quantization.config.MaxCalibConfig>` for
     details on the remaining arguments.
@@ -114,7 +119,7 @@ def max_calibrate(model: nn.Module, forward_loop: ForwardLoop | None = None, dis
         forward_loop(model)
     finish_stats_collection(model)
 
-    # Sync amax across local experts within each rank (for SequentialMLP)
+    # Sync input_quantizer amax across local experts within each rank (for SequentialMLP)
     for name, module in model.named_modules():
         if hasattr(module, "layer_sync_moe_local_experts_amax"):
             module.layer_sync_moe_local_experts_amax()

modelopt/torch/quantization/plugins/megatron.py

@@ -575,26 +575,30 @@ def _setup(self):
             expert.linear_fc2.parallel_state = self.parallel_state
 
     def layer_sync_moe_local_experts_amax(self):
-        """Sync amax across local experts in a SequentialMLP.
+        """Sync input quantizer amax across local experts in a SequentialMLP.
 
-        Synchronize the amax values across local experts in a lyaer such that all local experts will
-        share the same amax. This function operates on a single rank and does not require distributed sync.
+        Ensures all experts have the same input quantizer amax.This function operates
+        on a single rank and does not require distributed sync.
 
         Distributed amax sync across EP and ETP (for RowParallel) happens in model_calib.max_calibrate().
         This function should be called before the distributed sync to ensure the amax values
         are synchronized across the layer first.
 
         Note:
             Because there are logic which calls collective communication based on whether amax is not None,
-            We need to garuantee that all experts must have amax. Otherwise, there will be deadlock
-            when synchroizing over EP since some ranks may have amax None and not calling the collective
+            We need to guarantee that all experts must have amax. Otherwise, there will be deadlock
+            when synchronizing over EP since some ranks may have amax None and not calling the collective
             communication.
         """
         # Collect amax from all local experts
         amax_dict = {}
         for expert in self.local_experts:
             for name, module in expert.named_modules():
-                if isinstance(module, TensorQuantizer) and module.amax is not None:
+                if (
+                    isinstance(module, TensorQuantizer)
+                    and module.amax is not None
+                    and "input_quantizer" in name
+                ):
                     stored_amax = amax_dict.get(name)
                     amax_tensor = module.amax.detach().clone()
                     amax_dict[name] = (

tests/gpu_megatron/torch/quantization/plugins/test_megatron.py

@@ -469,10 +469,7 @@ def test_homogeneous_sharded_state_dict(tmp_path, config, compress, meta_device,
 
 @pytest.mark.parametrize(
     "config",
-    [
-        NVFP4_GEMM_KV_CFG,
-        FP8_GEMM_KV_CFG,
-    ],
+    [NVFP4_GEMM_KV_CFG, FP8_GEMM_KV_CFG, mtq.MAMBA_MOE_NVFP4_CONSERVATIVE_CFG],
 )
 def test_homogeneous_sharded_state_dict_hybrid(tmp_path, config):
     """Test sharded state dict for hybrid Mamba MOE models."""
@@ -731,6 +728,81 @@ def test_te_grouped_vs_sequential_quantize(need_4_gpus):
     )
 
 
+@pytest.mark.parametrize("ep_size", [1, 2])
+@pytest.mark.parametrize("moe_grouped_gemm", [True, False])
+def test_layer_sync_moe_local_experts_amax(ep_size, moe_grouped_gemm):
+    """Test expert model parallel synchronization."""
+    size = torch.cuda.device_count()
+    if size < ep_size:
+        pytest.skip(f"Requires at least {ep_size} GPUs for expert model parallel test")
+
+    spawn_multiprocess_job(
+        size=size,
+        job=partial(
+            _test_layer_sync_moe_local_experts_amax,
+            ep_size,
+            moe_grouped_gemm,
+        ),
+        backend="nccl",
+    )
+
+
+def _test_layer_sync_moe_local_experts_amax(ep_size, moe_grouped_gemm, rank, size):
+    initialize_for_megatron(
+        tensor_model_parallel_size=1,
+        pipeline_model_parallel_size=1,
+        expert_model_parallel_size=ep_size,
+        expert_tensor_parallel_size=1,
+        seed=SEED,
+    )
+    model = _gpt_model_provider(
+        tp_size=1,
+        ep_size=ep_size,
+        etp_size=1,
+        hidden_size=256,
+        moe_grouped_gemm=moe_grouped_gemm,
+        use_te=moe_grouped_gemm,
+        num_moe_experts=8,
+        transformer_impl="modelopt",
+    )
+    quant_cfg = mtq.FP8_DEFAULT_CFG
+    model = mtq.quantize(model, quant_cfg, get_forward(model))
+
+    for layer in model.decoder.layers:
+        layer.mlp.experts.layer_sync_moe_local_experts_amax()
+
+    for layer in model.decoder.layers:
+        # Check input quantizer amax is synced across local experts
+        fc1_amax = None
+        fc2_amax = None
+        for expert in layer.mlp.experts.local_experts:
+            assert expert.linear_fc1.input_quantizer.amax is not None
+            assert expert.linear_fc2.input_quantizer.amax is not None
+            if fc1_amax is None:
+                fc1_amax = expert.linear_fc1.input_quantizer.amax
+            else:
+                assert torch.allclose(fc1_amax, expert.linear_fc1.input_quantizer.amax)
+            if fc2_amax is None:
+                fc2_amax = expert.linear_fc2.input_quantizer.amax
+            else:
+                assert torch.allclose(fc2_amax, expert.linear_fc2.input_quantizer.amax)
+
+        # Check weight quantizer amax is different across local experts
+        fc1_amax = None
+        fc2_amax = None
+        for expert in layer.mlp.experts.local_experts:
+            assert expert.linear_fc1.weight_quantizer.amax is not None
+            assert expert.linear_fc2.weight_quantizer.amax is not None
+            if fc1_amax is None:
+                fc1_amax = expert.linear_fc1.weight_quantizer.amax
+            else:
+                assert not torch.allclose(fc1_amax, expert.linear_fc1.weight_quantizer.amax)
+            if fc2_amax is None:
+                fc2_amax = expert.linear_fc2.weight_quantizer.amax
+            else:
+                assert not torch.allclose(fc2_amax, expert.linear_fc2.weight_quantizer.amax)
+
+
 def _test_expert_model_parallel_amax_sync(
     tp_size, ep_size, etp_size, moe_grouped_gemm, config, rank, size
 ):
@@ -811,9 +883,6 @@ def test_expert_parallel_sync(config, ep_size, etp_size, moe_grouped_gemm):
     if size < ep_size * etp_size:
         pytest.skip(f"Requires at least {ep_size * etp_size} GPUs for expert model parallel test")
 
-    if moe_grouped_gemm:
-        pytest.skip("TEGroupedMLP is not enabled in Megatron-LM currently")
-
     spawn_multiprocess_job(
         size=size,
         job=partial(