This updates CUDA fast math intrinsic lowering to use a PassContext
option instead of a CUDA Target attribute.
The new option is:
```python
with tvm.transform.PassContext(config={"tirx.enable_fast_math": True}):
...
```
When unset or false, CUDA math intrinsics continue to lower to the
precise CUDA math functions such as expf. When true, tirx.LowerIntrin
prioritizes the cuda.fastmath.* lowering rules, producing fast math
intrinsics such as __expf.
Shushi Hong
59bfb21559