Change merge feature to avoid looping through env.

Replacement for equivalent nodes and constants are now queued when the graph is modified, through the MergeFeature, and processed when the optimization is applied. This means that appling the MergeOptimization when there is nothing to be merged is now almost instantaneous, even for big graphs. It is also faster when lots of nodes have to be merged (empirical 40 % speed-up for a graph with ~ 5000 nodes).

Change merge feature to avoid looping through env.
3c818691 · Pascal Lamblin · a96d5716 · 3c818691 · 3c818691
--- a/theano/compile/mode.py
+++ b/theano/compile/mode.py
@@ -204,7 +204,7 @@ optdb.register('merge1', gof.MergeOptimizer(),
        0, 'fast_run', 'fast_compile')
 optdb.register('canonicalize', gof.EquilibriumDB(),         # rearranges elemwise expressions
        1, 'fast_run', 'fast_compile')
-optdb.register('merge1.2', gof.MergeOptimizer(skip_const_merge=False),
+optdb.register('merge1.2', gof.MergeOptimizer(),
        1.2, 'fast_run', 'fast_compile')
 optdb.register('Print1.21', PrintCurrentEnv('Post-canonicalize'),
        1.21,)# 'fast_run', 'fast_compile')

--- a/theano/gof/opt.py
+++ b/theano/gof/opt.py