DLPack: fix test using PyTorch API.

[ysiraichi]

This PR fixes the test introduced in #7213, where it called PyTorch's to_dlpack() function before actually calling from_dlpack(). Meaning that both Tensor.__dlpack__ and Tensor.__dlpack_device__ functions weren't getting called, which were the functions that that test was supposed to be testing.

This should be merged after pytorch/pytorch#138470.

cc @JackCaoG @vanbasten23 @qihqi

[ysiraichi]

At first, I won't add the .torch_pin file, so as to make sure the test is failing without pytorch/pytorch#138470. Then, I will add it, so as to make sure it is the reason the test passes.