Fill dtype defaults to 'float' in backend_wasm

[pabloegpf1]

To see the logs from the Cloud Build CI, please join either our discussion or announcement mailing list.

---

This change is 

Fill op defaults the "dtype" parameter to "float" according to the docs.

@param dtype
The type of an element in the resulting tensor. Defaults to 'float'.

However when switching to wasm backend you need to explicitly specify the parameter or it will show an error Unknown dtype ${dtype}