Add tensor slice indexing

[OrigamiDream]

This PR aims to provide a Slice(start, end, step). The following illustrates the usage.

use tch::{Tensor, IndexOp, Slice};

let tensor = Tensor::from_slice(&[1, 2, 3, 4, 5, 6]).view((2, 3));
let result = tensor.i((.., Slice::new(None, None, 2)));

It shows exactly same behavior in PyTorch: tensor[:, ::2].

I had considered several ways of using Slice, such as:

let _ = Slice(0, -1, 2); // all exactly `i64`
let _ = Slice(Some(0), None, 2); // uses `Option<i64>` for `start` and `end`
let _ = Slice::new(0, None, 2); // add `new` method and uses `impl Into<Option<i64>>`

I ended up using the third one, which feels more natural to me. But I hope to discuss about this design more.