FIX: If a transform function was provided, don't recompile it

This helps when the transform function uses it's own cache mechanism, e.g.
gramex.cache.open, and has its own cache. This ensures that the same
transform does not cause a new cache to be created.

gramex/data.py

@@ -1258,9 +1258,14 @@ def alter(
     return engine
 
 
+_transform_cache = {}
+
+
 def _transform_fn(transform, transform_kwargs):
     if callable(transform) and transform_kwargs is not None:
-        return lambda v: transform(v, **transform_kwargs)
+        if transform not in _transform_cache:
+            _transform_cache[transform] = lambda v: transform(v, **transform_kwargs)
+        return _transform_cache[transform]
     return transform