Limit loop unrolling (#120023)
Tacotron2 causes massive loop unrolling resulting in very large graphs (26k nodes) which was causing inductor (and tracing itself) to choke.
The unrolling size is controlled by the environment variable TORCHDYNAMO_MAX_LOOP_UNROLL_NODES which defaults to the arbitrary value 5000.
This updates the tacotron2 timings as follows:
eager timing: 3m:23s -> 35s
aot_eager timing: 4m:12s -> 39s
inductor timing: 22m:24s ->1m
For reference the big loop in tacotron2 was this one (model.py[405]):
```
while len(mel_outputs) < decoder_inputs.size(0) - 1:
decoder_input = decoder_inputs[len(mel_outputs)]
mel_output, gate_output, attention_weights = self.decode(decoder_input)
mel_outputs += [mel_output.squeeze(1)]
gate_outputs += [gate_output.squeeze(1)]
alignments += [attention_weights]
```
which gets unrolled and inlined adding about 36 nodes to the graph per iteration.
Fixes #98467
Relates to #102839 which hopefully will result in a better fix.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120023
Approved by: https://github.com/yanboliang
