Deprecate old QAT APIs

**Summary:** Deprecates QAT APIs that should no longer be used.
Print helpful deprecation warning to help users migrate.

**Test Plan:**
```
python test/quantization/test_qat.py -k test_qat_api_deprecation
```

Also manual testing:
```
'IntXQuantizationAwareTrainingConfig' is deprecated and will be removed in a future release. Please use the following API instead:

    base_config = Int8DynamicActivationInt4WeightConfig(group_size=32)
    quantize_(model, QATConfig(base_config, step="prepare"))
    # train (not shown)
    quantize_(model, QATConfig(base_config, step="convert"))

Alternatively, if you prefer to pass in fake quantization configs:

    activation_config = IntxFakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
    weight_config = IntxFakeQuantizeConfig(torch.int4, group_size=32)
    qat_config = QATConfig(
        activation_config=activation_config,
        weight_config=weight_config,
        step="prepare",
    )
    quantize_(model, qat_config)

Please see #2630 for more details.

IntXQuantizationAwareTrainingConfig(activation_config=None, weight_config=None)
```

ghstack-source-id: 4e209aa
Pull Request resolved: #2641

Author: andrewor14

docs/source/api_ref_qat.rst

@@ -35,15 +35,13 @@ Custom QAT APIs
     linear.enable_linear_fake_quant
     linear.disable_linear_fake_quant
 
-Legacy QAT APIs
+Legacy QAT Quantizers
 ---------------------
 
 .. autosummary::
     :toctree: generated/
     :nosignatures:
 
-    IntXQuantizationAwareTrainingConfig
-    FromIntXQuantizationAwareTrainingConfig
     Int4WeightOnlyQATQuantizer
     linear.Int4WeightOnlyQATLinear
     Int8DynActInt4WeightQATQuantizer

torchao/quantization/qat/fake_quantize_config.py

@@ -35,6 +35,8 @@
 
 from .utils import _log_deprecation_warning
 
+from .utils import _log_deprecation_warning
+
 
 class FakeQuantizeConfigBase(abc.ABC):
     """