better dispose scope

[yueyinqiu]

The current dispose scope system does not support async and may have other problems when being misused by users.

So would it be a better idea to have something like:

using var scope = new Scope();
var x = scope.Tensor(0);

And things like y = x + 1 could automatically attach y to the same scope of x. (And tensor(1) should be directly disposed after the operation.) Hmm... and for z = x + y... maybe we should dispose z when x.scope and y.scope are both disposed.

Actually I got the idea from a comment of this post, and I think the only reason we don't do this is that it may lead to inconsistency with pytorch. But I still believe this is a really good idea.