Make efficientnet trainer fast

[geohot]

• [x] Support asymmetric padding
• [ ] Support caching of only unresolved LazyBuffers See CACHE_LAZYBUFFERS
• [ ] Support not running the convs multiple times, aka late split. This fixes an openpilot runner bug too

# <enum 'ProcessingOps'> 70
GRAPH=1 CNT=1 GPU=1 PYTHONPATH="." python3 examples/benchmark_train_efficientnet.py
# <enum 'ProcessingOps'> 208
GRAPH=1 CNT=1 LAZY=1 PYTHONPATH="." python3 examples/benchmark_train_efficientnet.py

• [x] Support counting FLOPS and work out theoretical max
• [x] "Upstream" LAZY and make it default
• [ ] Remove padding from the ml conv op and remove the slice on execution?
• [ ] Collapse reduce into binary op
• [ ] Improve the "JIT" to not constant fold things like LR.