[reland] Move QAT out of prototype

[andrewor14]

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

(Future) BC-breaking notes

Note: This commit itself doesn't break BC yet. A future PR will do that. The following is just to save this BC breaking note somewhere.

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1152

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• [PRE-EMPTIVE] Experimenting with new runners linux.aws.a100 on inductor-perf-compare.yml

❌ 1 New Failure

As of commit e2dd867 with merge base 7aaf0ff ():

NEW FAILURE - The following job has failed:

• Run Regression Tests / test (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://download.pytorc... / linux-job (gh)
  test/sparsity/test_sparse_api.py::TestQuantSemiSparse::test_quant_semi_sparse_compile_True

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