[feature request] function decorator `timer` should support direct call

Currently, it can only be
```
@timer()
def my_func():
pass
```
But, the following code is expected to run but fail:
```
@timer
def my_func():
pass
```
An fix can be easily done to decorator definition by
```
def example2(_func=None, *, kw1=val1, kw2=val2):
if _func is None:
return functools.partials(example2, kw1=val1, kw2=kw2)

@functools.wraps(func)
def wrapper(*args, **kwargs)：
# do things with kw1
return func(*args, **kwargs)
return wrapper
```
I am willing to make a PR if it is needed.