Unbreak BC, add back PerRow, PerTensor imports (#3396)

**Summary:** OSS users like sglang are still importing these
from `torchao.quantization.observer`. Here we quickly unbreak
BC since that was not the intention of
#3370.

**Test Plan:** CI

Author: andrewor14

torchao/quantization/observer.py

@@ -12,7 +12,7 @@
 
 from torchao.quantization.quant_primitives import _fake_quantize_affine
 
-from .granularity import Granularity
+from .granularity import Granularity, PerRow, PerTensor  # noqa: F401
 from .quant_primitives import (
     MappingType,
     ZeroPointDomain,