make weights loading more robust by using bounds-checked accessor

[fps]

.at() has the property of being bounds-checked and that an exception is thrown in the out-of-bounds case.

[mikeoliphant]

Doing .at() for each weight is likely adding overhead. Might not be significant, but I think it makes more sense to do manual bounds checking on blocks.

[fps]

Yes, the bounds checks add overhead, but even if it's 100 times slower, it doesn't really matter when the number of weights is 13000 or so. Maybe it will take 1 ms instead of 10 us on a core 2 duo class cpu ;)

https://www.w3tutorials.net/blog/accessing-elements-of-a-vector-in-c/#performance-comparison-speed-vs-overhead

And the weights loading is not in the time critical DSP path anyways.

[mikeoliphant]

Model load performance can be a consideration on very low powered machines, but it is more of a matter of principle than anything else. Why check bounds 100s of times when you can do it once? Plus, you can then throw a more informative error than a generic vector bounds exception.

[fps]

OK! We'll have to agree to disagree then.

[mikeoliphant]

Addressed via 14fb7bf