[CUDA] Fix FP16 Precision for Sigmoid Op (#14727)
Current Sigmoid's CUDA kernel uses target data type for all computation.
For some small negative numbers, if using FP16, it will loss precision.
For example, for input [-7.8477, 7.3320, -7.8008, 6.6016], the expected
output is [3.9047e-04, 9.9935e-01, 4.0919e-04, 9.9864e-01], but current
kernel will generate result [0.0000, 0.9990, 0.0000, 0.9990]. If some
sub-graph contains Sigmoid, such as BinaryCrossEntropyWithLogits, it's
likely to produce NaN as compute result.
The PR fixes this by using FP32 for kernel internal computation. Note
that the fix will not have perf regression, as CUDA's _Exp will also do
float to half casting, so the fix doesn't introduce extra cast. We move
the cast to right begin and end of the whole kernel so that other parts
of computation are also in FP32 (instead of only Exp).
