* Improve Variable interface
* Address comments from @apaszke and @colesbury
* string ::operator= is not noexcept
* Remove ir.h from tracer_state.h to improve build times
* Make Variable a struct and pack SavedVariable fields
* Implement as_variable_ref
* grad_fn_ptr() -> grad_fn_unsafe()
* Reduce hackiness of set_type hack
* Include variable.h and edge.h in tracer_state.h because it uses them
* class Variable -> struct Variable because Windows cant even
* Make Variable::output_nr uint32_t instead of int
* Add comment about tracing state
* Replaced more static_cast<Variable&> and improve docs
* Remove SavedVariable destructor and construct members in init list
* Clarify docs for Variable
* Variable::set_version -> set_version_counter
