provide opaque tensor type `ttl::Tensor`

[lgarithm]

The class ttl::Tensor should have no template parameters.

discussions:

• Q: What will be the relationship between std::Tensor and std::raw_tensor?
  A: currently raw_tensor only hides dtype and rank but not device

• A: should we use ttl::raw_tensor instead of ttl::Tensor?

• Q: what else does ttl::Tensor provide?
  A: a). tensor with non-consecutive storage. b). tensor with customized strides, c) sparse tensor?

[lgarithm]

The Tensor type is currently implemented under stdml namespace.

[lgarithm]

should the opaque tensor be under the namespace ttl? e.g. ttl::opaque_tensor (without any template parameters)