Update on "Move QAT out of prototype"

**Summary:** Move QAT out of prototype so we can provide stronger
BC guarantees moving forward.

**BC-breaking notes**

Before:
```
from torchao.quantization.prototype.qat import (
    disable_4w_fake_quant,
    disable_8da4w_fake_quant,
    enable_4w_fake_quant,
    enable_8da4w_fake_quant,
    ComposableQATQuantizer,
    Int4WeightOnlyQATQuantizer,
    Int4WeightOnlyEmbeddingQATQuantizer
    Int8DynActInt4WeightQATQuantizer,
    Int8DynActInt4WeightQATLinear,
)
from torchao.quantization.prototype.qat.api import (
    FakeQuantizeConfig,
)
from torchao.quantization.prototype.qat.fake_quantizer import (
    FakeQuantizer,
)
```

After:
```
from torchao.quantization.qat import (
    ComposableQATQuantizer,
    Int4WeightOnlyQATQuantizer,
    Int4WeightOnlyEmbeddingQATQuantizer
    Int8DynActInt4WeightQATQuantizer,
)
from torchao.quantization.qat.linear import (
    disable_4w_fake_quant,
    disable_8da4w_fake_quant,
    enable_4w_fake_quant,
    enable_8da4w_fake_quant,
    Int8DynActInt4WeightQATLinear,
)
from torchao.quantization.qat.api import (
    FakeQuantizeConfig,
)
from torchao.quantization.qat.fake_quantizer import (
    FakeQuantizer,
)
```

**Test Plan:**
python test/quantization/test_qat.py

Differential Revision: [D64555609](https://our.internmc.facebook.com/intern/diff/D64555609)

[ghstack-poisoned]

Author: andrewor14