Export `TensorIndexer to candle users

[h1994st]

This pull request exports TensorIndexer to candle lib users, which allows users to implement the IndexOp trait with TensorIndexer specified as a part of the constraints.

pub struct CustomStruct {
    data: Tensor,
    // ...
}

impl<A> IndexOp<A> for CustomStruct
where
    A: Into<TensorIndexer>,
{
    fn i(&self, index: A) -> Result<Tensor, Error> {
        self.data.i(index)
    }
}