title: Pydantic default value from other class' field
body: <p>Is there any chance to set as default value value from other class' field:</p>
  <pre><code>class Data(BaseModel):
      field_1: Optional[str] = Data2.field_2
  
  
  class Data2(BaseModel):
      field_2: Optional[str] = ''
  </code></pre>
  <p>I've tried with:</p>
  <pre><code>Data2 = ForwardRef('Data2')
  .
  .
  .
  Data.update_forward_refs()
  </code></pre>
  <p>getting: cannot encode object: ForwardRef('Data2'), of type: &lt;class
  'typing.ForwardRef</p>