requirements.txt: bump autograd dependency to 1.2
requirements.txt: bump autograd dependency to 1.2

Autograd 1.2 introduced the function 'hessian_tensor_product' so we
don't have to implement it by hand.

Signed-off-by: Niklas Koep <342d5290239d9c5264c8f98185afedb99596601a@gmail.com>

nkoep/pymanopt,nkoep/pymanopt,nkoep/pymanopt,pymanopt/pymanopt,pymanopt/pymanopt