Fix two problems in libdevice and external dispatch:
1. Use static triton types (e.g., tl.int32) instead of creating new
types. Otherwise, `tl.int32` and `tl.dtype('int32')` are not the same
thing.
2. The name of an extern inst should be empty but not the symbol name of
the inst. TTIR generator will assign names automatically. Otherwise, we
have the same variable name when there are multiple same extern insts.
Before the PR:
```bash
__nv_exp = extern_elementwise f64<1024> %11;
__nv_exp = extern_elementwise f64<1024> %11;
```
After the PR:
```bash
%12 = extern_elementwise f64<1024> %11;
%13 = extern_elementwise f64<1024> %11;
```
db3aa1d1fb