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# apaszke/jacobian_hessian.py

Last active Sep 12, 2020

### KhalilElkhalil commented Jan 17, 2020

 Dear Adam, Is there a way to compute the Laplacian of a function f w.r.t a tensor x with dimension bxD (b: batch size, D: data dimension)? We need to compute $\sum_{i=1}^D \partial^2 f(x) / \partial x_i^2$ in an efficient way. Computing the Hessian and taking the trace seems to compute unnecessary off-diagonals which are irrelevant to the Laplacian. Thanks a lot!

### apaszke commented Jan 25, 2020

 I don't think there's any other way with the current AD methods. You don't have to keep the whole Hessian in memory of course (you can throw away a row of the Hessian once you've picked out the element you're interested in), but you'll still need to compute each row, just like the hessian function does.

### slerman12 commented Apr 18, 2020

 Is there a reason why you use grad_y instead of just indexing flat_y[i] in the autograd?

### jalane76 commented Apr 19, 2020

 Hello, I am relatively new to PyTorch and came across your Hessian function. It is much more elegant than some Hessian code from an academic paper that I am trying to reproduce. I've put together a toy example, but keep getting the error RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.FloatTensor ] is at version 4; expected version 3 instead. Hint: the backtrace further above shows the operation that failed to compute its gradient. The variable in question was changed in there or anywhere later. Good luck! I've been scouring the docs and googling, but for the life of me I can't figure out what I'm doing wrong. Any help you could offer would be greatly appreciated! Here is my code: import torch import torch.autograd as autograd import torch.nn as nn import torch.nn.functional as F import torch.optim as optim import numpy as np torch.set_printoptions(precision=20, linewidth=180) def jacobian(y, x, create_graph=False): jac = [] flat_y = y.reshape(-1) grad_y = torch.zeros_like(flat_y) for i in range(len(flat_y)): grad_y[i] = 1. grad_x, = torch.autograd.grad(flat_y, x, grad_y, retain_graph=True, create_graph=create_graph) jac.append(grad_x.reshape(x.shape)) grad_y[i] = 0. return torch.stack(jac).reshape(y.shape + x.shape) def hessian(y, x): return jacobian(jacobian(y, x, create_graph=True), x) def f(x): return x * x np.random.seed(435537698) num_dims = 2 num_samples = 3 X = [np.random.uniform(size=num_dims) for i in range(num_samples)] mean = torch.Tensor(np.mean(X, axis=0)) mean.requires_grad = True cov = torch.Tensor(np.cov(X, rowvar=False)) with autograd.detect_anomaly(): hessian_matrices = hessian(f(mean), mean) print('hessian: \n{}\n\n'.format(hessian_matrices))  The output with anomaly detection turned on is here: RuntimeError Traceback (most recent call last) in () 67 68 with autograd.detect_anomaly(): ---> 69 hessian_matrices = hessian(f(mean), mean) 70 print('hessian: \n{}\n\n'.format(hessian_matrices)) 2 frames in hessian(y, x) 45 print('--> hessian()') 46 j = jacobian(y, x, create_graph=True) ---> 47 return jacobian(j, x) 48 49 def f(x): in jacobian(y, x, create_graph) 28 print('\tgrad_y: \n\t{}\n'.format(grad_y)) 29 ---> 30 grad_x, = torch.autograd.grad(flat_y, x, grad_y, retain_graph=True, create_graph=create_graph) 31 print('\tgrad_x: \n\t{}\n') 32 /usr/local/lib/python3.6/dist-packages/torch/autograd/init.py in grad(outputs, inputs, grad_outputs, retain_graph, create_graph, only_inputs, allow_unused) 155 return Variable._execution_engine.run_backward( 156 outputs, grad_outputs, retain_graph, create_graph, --> 157 inputs, allow_unused) 158 159 RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.FloatTensor ] is at version 4; expected version 3 instead. Hint: the backtrace further above shows the operation that failed to compute its gradient. The variable in question was changed in there or anywhere later. Good luck! Finally, I'm running my code in a Google Colab notebook with PyTorch 1.4 if that makes a difference. Thanks!

### jalane76 commented Apr 20, 2020

 I did manage to get the code to run now. I made a "simplification" that broke it. Your function f is: def f(x): return x * x * torch.arange(4, dtype=torch.float)  While mine was: def f(x): return x * x  I've since fixed it to: def f(x): return x * x * torch.ones_like(x)  and it works like a charm. @apaszke any idea why that is the case?

### el-hult commented Apr 20, 2020

 I did manage to get the code to run now. I made a "simplification" that broke it. Your function f is: def f(x): return x * x * torch.arange(4, dtype=torch.float)  While mine was: def f(x): return x * x  I've since fixed it to: def f(x): return x * x * torch.ones_like(x)  and it works like a charm. @apaszke any idea why that is the case? you can switch torch.ones_like(x) to 1 and it still works...

### Ronnypetson commented Jun 26, 2020

 Hello Adam! How could I give credit to you if I use this code? Can it be a doc-string in documentation, paper citation or something?