How can I use Feast with only InputRequest source

[htluandc2]

Is your feature request related to a problem? Please describe.
I can't use only RequestSource when make new features.

Describe the solution you'd like
Hi there,
I'm on a competition programming. I want to make some new features for my model. But I can't only using RequestSource on FeatureService. I must have least one FeatureView for FeatureService.

Describe alternatives you've considered
In short term, I make an FeatureView with one dummy column, like it:

fv = FeatureView(
    name='fv',
    entities=[entity],
    source=source,
    schema=[
         # I maked an dummy column 
        Field(name="dummy", dtype=feast.types.Float32),
    ]
)

features_v1 = FeatureService(
    name="features_v1",
    features=[
        fv,
        make_num_of_feature1,
    ]
)

But this solution is very slow in my competition.

Additional context
My code:

features_v1 = FeatureService(
    name="features_v1",
    features=[
        # fv,
        make_num_of_feature1,
    ]
)

My issue when I applied only InputRequest on FeatureSerivce:

f, validation_reference, timeout)
     64 def to_df(
     65     self,
     66     validation_reference: Optional["ValidationReference"] = None,
     67     timeout: Optional[int] = None,
     68 ) -> pd.DataFrame:
     69     """
     70     Synchronously executes the underlying query and returns the result as a pandas dataframe.
     71 
   (...)
     77         timeout (optional): The query timeout if applicable.
     78     """
---> 79     features_df = self._to_df_internal(timeout=timeout)
     81     if self.on_demand_feature_views:
     82         # TODO(adchia): Fix requirement to specify dependent feature views in feature_refs
...
   5900 ):
   5901     return self[name]
-> 5902 return object.__getattribute__(self, name)

AttributeError: 'DataFrame' object has no attribute 'persist'