Support varying numbers of residuals in autodiff.

This commit modifies the only function in autodiff that takes a
templated number of outputs (i.e. residuals) and makes that
template parameter a normal parameter. With that change, it
is a trivial matter to support a dynamic number of residuals.

The API for dynamic residuals is to pass a fake number of
residuals as the second template argument to
AutoDiffCostFunction, and to pass the real number of
parameters as a second constructor argument.

include/ceres/types.h

@@ -237,6 +237,13 @@ enum DumpFormatType {
   TEXTFILE
 };
 
+// For SizedCostFunction and AutoDiffCostFunction, DYNAMIC can be specified for
+// the number of residuals. If specified, then the number of residuas for that
+// cost function can vary at runtime.
+enum DimensionType {
+  DYNAMIC = -1
+};
+
 const char* LinearSolverTypeToString(LinearSolverType type);
 const char* PreconditionerTypeToString(PreconditionerType type);
 const char* LinearSolverTerminationTypeToString(