psv4

I made a pull request for fixed step stiff solvers in #265 . Once that's reviewed, I could start on adaptive stiff solvers

## Tests ``` python tests/run_all.py ........\torchdiffeq\torchdiffeq\torchdiffeq\_impl\rk_common.py:554: UserWarning: Functional iteration did not converge. Solution may be incorrect. warnings.warn('Functional iteration did not converge. Solution may be incorrect.') \torchdiffeq\torchdiffeq\torchdiffeq\_impl\rk_common.py:464: UserWarning: Functional iteration did...

@rtqichen this was a bigger update. For my work, the Jacobian inverse would take too much memory if it were dense. Following [this paper](https://arxiv.org/abs/2501.17737), I switched it to sparse and...

I had to do something similar. I followed the format from the (ANODE)[https://arxiv.org/abs/1904.01681] paper appending the additional parameters to the function vector. The difference is I make sure the derivative...

[Dopri5 Tableau](https://en.wikipedia.org/wiki/List_of_Runge%E2%80%93Kutta_methods#Dormand%E2%80%93Prince) Depending on your method, the forward function can be called multiple times for each integration. Starting with the simplest case of explicit Euler, your forward function is called...