Replace __str__ with __repr__ in some places (#136316)

Summary:
## The problem

In a typical debugger, `repr()` is used to display variables and not `str()`.

Several classes in Dynamo have a `__str__()` method that returns useful information and a  `__repr__()` that does not. Having to call `str(x)` or `[str(i) for i in x]` in the debugger all the time is a chore.

`str()` should be ["informal, nicely printable"](https://docs.python.org/3/library/stdtypes.html#str) and `repr()` should ["attempt to return a string that would yield an object with the same value when passed to eval()](https://docs.python.org/3/library/functions.html#repr)".

## The solution

In the Python object model, if there is no `__str__` method, `__repr__`  is used instead (but not the other way around).

So renaming `__str__` to `__repr__` in a few cases where no `__repr__` method exists now should not change observable behavior, and should make debugging easier.

The specific classes changed were all in `torch._dynamo.variables`:

* `builtin.BuiltinVariable`
* `constant.ConstantVariable`
* `constant.EnumVariable`
* `functions.UserMethodVariable`
* `lazy.LazyVariableTracker`
* `lazy.LazySymNodeFormatString`
* `misc.GetAttrVariable`
* `misc.NullVariable`
* `user_defined.UserDefinedObjectVariable`

X-link: pytorch/pytorch#136316
Approved by: https://github.com/XuehaiPan, https://github.com/jansel

Reviewed By: wdvr

Differential Revision: D64714511

fbshipit-source-id: 322f2f0110e5b45afe6a27c52a0bcc91d91d1d6a

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -2479,7 +2479,7 @@ def __init__(self, f):
         self.f = f
         self.__name__ = "wrapped_" + self.f.__name__
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<Wrapped function <original {self.f.__name__}>>"
 
     def __call__(self, *args, **kwargs):
@@ -2503,7 +2503,7 @@ def __init__(self, method: str):
         self.method = method
         self.__name__ = "wrapped_" + self.method
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<Wrapped method <original {self.method}>>"
 
     def __call__(self, *args, **kwargs):
@@ -2522,7 +2522,7 @@ def __init__(self, op: Callable[..., Any]):
         self.op = op
         self.__name__ = f"wrapped_{op.__name__}"
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<Wrapped operator <original {self.__name__}>>"
 
     def __call__(self, *args, **kwargs):
@@ -3097,7 +3097,7 @@ class Lit:
     def __init__(self, s):
         self.s = s
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return self.s