Before PyTorch there was Torch (which was Lua-based, hence the Py- prefix of the follow on PyTorch). With Torch, like the later TensorFlow 1.0, you first explic

Before PyTorch there was Torch (which was Lua-based, hence the Py- prefix of the follow on PyTorch). With Torch, like the later TensorFlow 1.0, you first explicitly constructed the computational graph then executed it. PyTorch's later innovation was define-by-run where you just ran the code corresponding to your computational graph and the framework traced your code and built the graph behind the scenes... this was so much more convenient that PyTorch quickly became more popular than TensorFlow, and by the time TF 2.0 "eager mode" copied this approach it was too late (although TF shot itself in the foot in many ways, which also accelerated it's demise).