Fix deprecation warnings with new Nx

[seanmor5]

Cleaning up final blockers before next release

[josevalim]

The warnings for Nx.tensor should be fixed by passing those as arguments instead of opts. That's easy for everything except for layers. I assume we don't want to make the opts[:rate] an argument, right? So the current fix is fine. I will push a commit for the initializers ones. :)

[josevalim]

I wonder if we should implement constant as literally:

def constant(init_value, _opts \\ []) do
  Nx.tensor(init_value)
end

?

[josevalim]

Besides the two minor comments, it looks good to me. I pushed a commit to address the initializers bit. :)