[LTC] Tweak LazyTensor Class for XLATensor
Summary:
This pull request makes some tweaks on LazyTensor class such that it's
easier for XLATensor to inherit.
1. It replaces data_ptr() with data() which now returns a const
shared_ptr& type.
2. It adds a temporary ctor to LazyTensor::Data such that XLATensor::Data
can easily inherits it.
3. It moves LazyTensor(std::shared_ptr<Data>) and SetTensorData(at::Tensor)
to protected for XLATensor to access.
Test Plan:
CI.
