Support String output tensors #67
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…rom a tensor This is some prep work for string output types and tensor types that vary across the model outputs. For now, the supported types are just the basic numeric types. Since strings have to be copied out of a tensor, it only makes sense to have `String` be an output type, not `str`, hence the new type so that we can have more input types supported than output types.
Outputs aren't all the same type for a single model, so this allows extracting different types per tensor.
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This approach allocates owned Strings for each element, which works, but stresses the allocator, and incurs unnecessary copying. Part of the complication stems from the limitation that in Rust, a field can't be a reference to another field in the same struct. This means that having a Vec<u8> of copied data, referred to by a Vec<&str>, which is then referred to by an ArrayView, requires a sequence of 3 structs to express. Building a Vec<String> gets rid of the references, but also loses the efficiency of 1 allocation with strs pointing into it.
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Based on #65.
This approach allocates owned Strings for each element, which works, but stresses the allocator, and incurs unnecessary copying.
Part of the complication stems from the limitation that in Rust, a field can't be a reference to another field in the same struct. This means that having a Vec of copied data, referred to by a Vec<&str>, which is then referred to by an ArrayView, requires a sequence of 3 structs to express. Building a Vec gets rid of the references, but also loses the efficiency of 1 allocation with strs pointing into it.
I'm not terribly happy with this implementation's efficiency but it does work, so I figured it was a decent starting point for later improvements. I'm playing with &str output in another branch but I haven't gotten the lifetimes to work out yet.