Very slow xvector computation with all time spend on compilation

While running Voxceleb with different architectures I noticed that xvector extraction is very slow:

```
nnet3-xvector-compute --verbose=0 --use-gpu=no --min-chunk-size=25 --chunk-size=10000 \
--cache-capacity=64 "nnet3-copy \
--nnet-config=exp/xvector_nnet_1a/extract.config \
exp/xvector_nnet_1a/final.raw - |" "ark:apply-cmvn-sliding \
--norm-vars=false --center=true --cmn-window=300 \
scp:feats.scp ark:- | select-voiced-frames \
ark:- scp,s,cs:data/voxceleb1_test/split20/1/vad.scp ark:- |" \
ark,scp:exp/xvector_nnet_1a/xvectors_voxceleb1_test/xvector.1.ark,exp/xvector_nnet_1a/xvectors_voxceleb1_test/xvector.1.scp
...
LOG (nnet3-xvector-compute[5.5.802~1-8d0c8]:main():nnet3-xvector-compute.cc:182) Chunk size of 10000 is greater than the number of rows in utterance: id10270-5r0dWxy17C8-00008, using chunk size of 1136
LOG (nnet3-xvector-compute[5.5.802~1-8d0c8]:main():nnet3-xvector-compute.cc:182) Chunk size of 10000 is greater than the number of rows in utterance: id10270-5r0dWxy17C8-00009, using chunk size of 812
LOG (nnet3-xvector-compute[5.5.802~1-8d0c8]:main():nnet3-xvector-compute.cc:182) Chunk size of 10000 is greater than the number of rows in utterance: id10270-5r0dWxy17C8-00010, using chunk size of 456
LOG (nnet3-xvector-compute[5.5.802~1-8d0c8]:main():nnet3-xvector-compute.cc:182) Chunk size of 10000 is greater than the number of rows in utterance: id10270-5r0dWxy17C8-00011, using chunk size of 420
....

LOG (nnet3-xvector-compute[5.5.669~1-b1d80]:main():nnet3-xvector-compute.cc:182) Chunk size of 10000 is greater than the number of rows in utterance: id10270-5r0dWxy17C8-00019, using chunk size of 764
LOG (select-voiced-frames[5.5.669~1-b1d80]:main():select-voiced-frames.cc:106) Done selecting voiced frames; processed 19 utterances, 0 had errors.
LOG (nnet3-xvector-compute[5.5.669~1-b1d80]:main():nnet3-xvector-compute.cc:238) Time taken 15.0148s: real-time factor assuming 100 frames/sec is 0.108457
LOG (nnet3-xvector-compute[5.5.669~1-b1d80]:main():nnet3-xvector-compute.cc:241) Done 19 utterances, failed for 0
LOG (nnet3-xvector-compute[5.5.669~1-b1d80]:~CachingOptimizingCompiler():nnet-optimize.cc:710) 12.9 seconds taken in nnet3 compilation total (breakdown: 12.7 compilation, 0.0195 optimization, 0 shortcut expansion, 0.0045 checking, 2.86e-06 computing indexes, 0.108 misc.) + 0 I/O.
```

Note that from 15.0148s of execution 12.7 were spent on compilation. Profiler confirms the issue, only 10% is in actual neural network computation.

It seems to be related to variable length of the chunks because if I submit chunks of equal size with --min-chunk-size=400 --chunk-size=400, the computation is much faster and compilation is done only once.

I wonder what is the proper approach to speedup this:

1. Fix something internally inside compilation so it will not compute again and again
2. Cluster on chunks of fixed width (probably use 100 frames steps - 100, 200, ...10000). Then such compilations will be cached more effectively.
3. I see there is also nnet3-xvector-compute-batched, but it also suffers from this issue. Is it supposed to work faster?