[LTC] Mark Step Indicator (#76840)
Proposed solution for #76826
Basically adds a context which is only "active" when called in mark step. Any backend can then use this to check if within a mark step context.
I've also added an example warning in the TS backend so that we now see the following:
```python
>>> import torch
>>> import torch._lazy
>>> import torch._lazy.ts_backend
>>> torch._lazy.ts_backend.init()
>>> a = torch.tensor([1, 2, 3, 4], device="lazy")
>>> b = torch.tensor([5, 6, 7, 8], device="lazy")
>>> c = a + b
>>> c
[W ts_backend_impl.cpp:187] Compile outside of mark step
tensor([ 6, 8, 10, 12], device='lazy:0')
>>> d = a * b
>>> torch._lazy.mark_step()
>>> d
tensor([ 5, 12, 21, 32], device='lazy:0')
```
Though it was mainly for example and will be happy to remove if this warning is not desired.
Fixes #76826
CC: @wconstab @desertfire @henrytwo @ke1337
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76840
Approved by: https://github.com/desertfire
