impl Tape for Arc<Mutex<OwnedTape>>

[coreylowman]

Related to #833

[coreylowman]

@emchristiansen want to point your dependency to this branch and see how useful it is?

dfdx = { git = "https://github.com/coreylowman/dfdx.git", branch = "arc-mutex-tape" }

You'll have to use it like:

let mut grads = ...;
let tape = Arc::new(Mutex::new(OwnedTape::from(grads)));
x.put_tape(tape)