feat(mge/dtype): add int2 lowbit support and example

GitOrigin-RevId: 67c14ac959a9f2725360f79cd3838000aa5e35ea

python_module/megengine/_internal/dtype.py

@@ -25,6 +25,9 @@ _metadata_dict = {
     "qint32": _QuantDtypeMetadata(
         "QuantizedS32", "int32", False, -(2 ** 31), 2 ** 31 - 1,
     ),
+    # NOTE: int2 is not supported for model dump yet
+    "quint2": _QuantDtypeMetadata(None, "uint8", True, 0, 3),
+    "qint2": _QuantDtypeMetadata(None, "int8", False, -2, 1),
 }
 
 

python_module/megengine/quantization/__init__.py

@@ -13,6 +13,7 @@ from .qconfig import (
     QConfig,
     calibration_qconfig,
     ema_fakequant_qconfig,
+    ema_lowbit_fakequant_qconfig,
     min_max_fakequant_qconfig,
     tqt_quant_qconfig,
 )

python_module/megengine/quantization/qconfig.py

@@ -92,6 +92,15 @@ ema_fakequant_qconfig = QConfig(
     act_fake_quant=partial(FakeQuantize, dtype="qint8", narrow_range=False),
 )
 
+ema_lowbit_fakequant_qconfig = QConfig(
+    weight_observer=partial(MinMaxObserver, dtype="qint4", narrow_range=False),
+    act_observer=partial(
+        ExponentialMovingAverageObserver, dtype="qint4", narrow_range=False
+    ),
+    weight_fake_quant=partial(FakeQuantize, dtype="qint4", narrow_range=False),
+    act_fake_quant=partial(FakeQuantize, dtype="qint4", narrow_range=False),
+)
+
 calibration_qconfig = QConfig(
     weight_observer=partial(MinMaxObserver, dtype="qint8", narrow_range=True),
     act_observer=partial(HistogramObserver, dtype="qint8", narrow_range=False),