text: Title: Specify hyperparameter ranges for blocks
  Body: ### Feature Description
  We want to enable the users to specify the value ranges for any argument in the blocks.
  The following code example shows a typical use case.
  The users can specify the number of units in a DenseBlock to be either 10 or 20.
  
  ### Code Example
  <!---
  Please provide a code example for using that feature
  given the proposed feature is implemented.
  -->
  ```python
  import autokeras as ak
  from kerastuner.engine.hyperparameters import Choice
  
  input_node = ak.ImageInput()
  output_node = ak.DenseBlock(num_units=Choice("num_units", [10, 20]))(input_node)
  output_node = ak.ClassificationHead()(output_node)
  model = ak.AutoModel(input_node, output_node)
  ```
  
  ### Note
  <!---
  Why do we need the feature?
  -->
  Each pull request should only change one hyperparameter in one of the blocks.
  
  ### Solution
  <!---
  Please tell us how to implement the feature,
  if you have one in mind.
  -->
  Example pull requests are #1419 #1425 .
  Here are the steps to follow:
  1. You can just change any other argument in any other block supported by AutoKeras, as shown [here](https://github.com/keras-team/autokeras/blob/master/autokeras/__init__.py#L16-L38).
  2. Change the docstring. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL60-R61)
  3. Make sure you imported the module. `from kerastuner.engine import hyperparameters`.
  4. Change the typing of the argument. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL73-R74)
  5. Change the saving mechanism to serialized objects. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL95-R100)
  6. Change the loading mechanism to deserialized objects. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cR110)
  7. Change how we initialize the hyperparameter to self. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL80-R85) Copy from where it is originally defined. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL115)
  8. Change how we use it. [example](https://github.com/keras-team/autokeras/pull/1419/files#diff-7b757ddf51e45c0e02ad8de148d02fd2abd59d670f91d35cac71157e6684c13cL124-R129)