Implement TryFrom<Tensor> for primitive numeric types to avoid panic

[tqwewe]

Tensor currently implements From<i32>, From<i64>, From<f32>
Tensor currently implements From<Tensor> for: i32, i64, f32, ... for some common rust types, and panics if theres more than one item in the Tensor.

TryFrom trait for these types are not implemented, but would be very handy in cases where you care about handling the error.

[LaurentMazare]

Just to be sure, do you actually mean From<i32> i.e. converting an i32 to a Tensor or From<Tensor> for i32 i.e. converting a Tensor to an i32? My guess is that it's the latter as you mention this erroring when there are more than one item in the tensor.

[tqwewe]

Ah I'm sorry, you're right, I mean From<Tensor> and TryFrom<Tensor> for i32, i64, f32, etc.

[LaurentMazare]

I've started looking into this but because of specialization not being handled properly, it's tricky to have both the From and TryFrom traits available (more details here). That said, I feel that switching to the TryFrom version might be a good idea although it's a breaking change, and in general the operation should be more fallible by default rather than the other way around. See e.g. #395, it's probably worth thinking a bit more about it (and keen to hear if other people have thoughts) but the idea would be to first switch the From conversion to TryFrom both for scalar and slices, see #605 that starts doing this.

[LaurentMazare]

The change has been merged so closing this for now, feel free to re-open if further issues.