datamodel-code-generator: Does not inherit all the classes mentioned in `allOf`

Describe the bug It is not extending all the mentioned classes in the allOf attribute of the schema.

To Reproduce I am running this on the directory: https://github.com/AnalyticalGraphicsInc/czml-writer/tree/master/Schema. But it does not inherit all the classes as I mentioned above. For example, one of the schemas is as follows,

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "$id": "https://analyticalgraphicsinc.github.io/czml-writer/Schema/Color.json",
    "title": "Color",
    "description": "A color. The color can optionally vary over time.",
    "allOf": [
        { "$ref": "InterpolatableProperty.json" },
        { "$ref": "DeletableProperty.json" },
        { "$ref": "ValueProperties/RgbaValueProperty.json" },
        { "$ref": "ValueProperties/RgbafValueProperty.json" },
        { "$ref": "ValueProperties/ReferenceValueProperty.json" }
    ],
    "type": [
        "array",
        "object"
    ],
    "items": {
        "$ref": "#"
    },
    "properties": {
        "rgba": {
            "$ref": "Values/RgbaValue.json",
            "description": "The color specified as an array of color components `[Red, Green, Blue, Alpha]` where each component is an integer in the range 0-255."
        },
        "rgbaf": {
            "$ref": "Values/RgbafValue.json",
            "description": "The color specified as an array of color components `[Red, Green, Blue, Alpha]` where each component is a double in the range 0.0-1.0."
        },
        "reference": {
            "$ref": "Values/ReferenceValue.json",
            "description": "The color specified as a reference to another property."
        }
    }
}

It generates the class as follows:

# generated by datamodel-codegen:
#   filename:  Color.json
#   timestamp: 2021-03-29T05:46:25+00:00

from __future__ import annotations

from typing import Optional

from pydantic import BaseModel, Field

from . import Cylinder, LabelStyle


class Color(BaseModel):
    """
    A color. The color can optionally vary over time.
    """

    rgba: Optional[Cylinder.RgbaValue] = Field(
        None,
        description='The color specified as an array of color components `[Red, Green, Blue, Alpha]` where each component is an integer in the range 0-255.',
    )
    rgbaf: Optional[Cylinder.RgbafValue] = Field(
        None,
        description='The color specified as an array of color components `[Red, Green, Blue, Alpha]` where each component is a double in the range 0.0-1.0.',
    )
    reference: Optional[LabelStyle.ReferenceValue] = Field(
        None, description='The color specified as a reference to another property.'
    )

Expected behavior It should have inherited the classes DeletableProperty and InterpolatableProperty

Additional context Those classes have been generated and are residing in the same directory. It’s just that they are not being imported and inherited.