Check for dimension mismatch in param.

src/params/header.js

@@ -88,6 +88,14 @@ module.exports = function(env) {
     function finish(s) {
       var val = params.fetch(name, env);
       var valDims = ad.value(val).dims;
+      if (!_.isEqual(dims, valDims)) {
+        var msg = 'The dims specified here (' + JSON.stringify(dims) +
+            ') do not match the dims of the current value (' +
+            JSON.stringify(valDims) + '). The current value may ' +
+            'come from an earlier call to param, or from a previous ' +
+            'execution when a persistent parameter store is used.';
+        throw new Error(msg);
+      }
       return k(s, dims === dimsForScalarParam ? ad.tensor.get(val, 0) : val);
     };
   };