converting sympy.NumberSymbol to torch.tensor in export_torch.py

attempting to address #656

pysr/export_torch.py

@@ -116,7 +116,7 @@ def __init__(self, *, expr, _memodict, _func_lookup, **kwargs):
                     self._args = ()
                 elif issubclass(expr.func, sympy.NumberSymbol):
                     # Can get here from exp(1) or exact pi
-                    self._value = float(expr)
+                    self._value = torch.tensor(float(expr))
                     self._torch_func = lambda: self._value
                     self._args = ()
                 elif issubclass(expr.func, sympy.Symbol):