Fix `FbgemmFp8Linear` not preserving tensor shape (#33239)
* add tests for linear shape behavior
* fix linear shape behavior
ended up adding the reshape at the end, after f8f8bf16_rowwise, because adding
it directly after quantize_fp8_per_row caused f8f8bf16_rowwise to drop the
seq_len dimension. (i.e., (17, 23, 1014) -> (17, 1024))
* save shape up front + comment
