inputs is "float32", autocast casts all inputs to "float32" and runs
the op in "float32".
"cat", "stack", "index_copy"
Some ops not listed here (e.g., binary ops like "add") natively
promote inputs without autocasting's intervention.  If inputs are a
mixture of "bfloat16" and "float32", these ops run in "float32" and
produce "float32" output, regardless of whether autocast is enabled.