Fix successive comparisons in EXLA

[seanmor5]

Successive comparisons fail in EXLA because they always return {:pred, 8} types and the compare operator in turn relies on Nx.Type.merge to merge result types in a successive comparison. Test is one example of something that fails before this. Doing something like:

y_true
|> Nx.equal(1)
|> Nx.as_type({:u, 8})
|> Nx.equal(2)

I'm pretty sure would workaround this, but I think this issue might manifest in other places as well, so I figured this was an easier workaround.

[josevalim]

If we were calling this code, it would have been raising in the past, no?

[josevalim]

Oh, this is what you mean, it was failing. Yeah, today Nx.Type doesn't know about pred. I would prefer if it still did not. I will investigate.