[LTC] Get rid of current device concept (#68054)
Summary:
The current device concept is used in three scenarios, and each either now
has a better replacement or is not applicable. Here they are:
1) Used by UAPI _ltc_set_default_device/_ltc_get_default_device: these APIs
are available for users before but actually make no senses. It makes sense
to set default device type to CPU, GPU or TPU, but not lazy given that would
just use whatever default hardware type. Setting ordinal like lazy:0 doesn't
make any sense now as distributed training is still under development.
2) Used by LTCGuardImpl: it only needs to know c10 device but not backend
device. Replaced the g_device with c10 one.
3) Used as the default device: replaced with Device(), which later will
be enhanced with the default device type backend API.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc
