Fix error when destructing a static std::unique_ptr<cppflow::model>

[ihowell]

Fixes #131

When creating a static version of a reference to a cppflow::model,
there becomes an error when destructing, due to the
context::get_status() method using a thread_local status. While
this is done normally for performance gains and is fine in most
places. However, when the program is exiting, it first destructs thread_local
objects and then static ones. This leads to local_tf_status
containing a nullptr instead of a pointer to a TF_Status
object. Therefore, TF_DeleteSession(sess, status) fails because it
access *status. The simplest fix is to not re-use this status object
in the destructor of the session and instead make a new,
non-thread_local status instead.

[serizba]

Hi @ihowel, thanks for spotting this!

Perhaps is better to allocate on TF_Status per model, as suggested by @ljn917 here.

[ljn917]

@serizba Do you mind me having a PR to fix, i.e. adding a per model TF_Status? The global status will be used for eager ops only. There were so many people trying to build cppflow into a dll and having the deinitialuzation order fiasco.

[serizba]

@ljn917 Yeah sure! This way we'll fix all related issues.

Thanks!