logsumexp for multiple dimensions (#16475)

logsumexp for multiple dimensions (#16475)

Summary:
Move `logsumexp` and `max_values` to `TensorIterator` and use it to make `logsumexp` work for multiple dimensions.

Timings on a tensor of shape `(10,1000000,10)`, for each combination of (cpu, single-threaded cpu, gpu) and dimension:

**before**
208 ms ± 2.72 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
279 ms ± 5.07 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
199 ms ± 2.64 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
1.11 s ± 33.3 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
1.25 s ± 25.3 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
1.11 s ± 6.83 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
15.4 ms ± 1.02 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
132 ms ± 30.1 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
39.6 ms ± 19.1 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

**after**
199 ms ± 8.23 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
307 ms ± 8.73 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
207 ms ± 7.62 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)
1.16 s ± 8.92 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
1.26 s ± 47.6 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
1.13 s ± 13.7 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
15.4 ms ± 868 ns per loop (mean ± std. dev. of 7 runs, 100 loops each)
132 ms ± 27.6 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
39.6 ms ± 21.8 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16475

Differential Revision: D13855746

Pulled By: umanwizard

fbshipit-source-id: aaacc0b967c3f89073487e1952ae6f76b7bd7ad3