Same model returned by pydantic_model_creator calls with different arguments

[henadzit]

Description

This PR fixes the issue where calling pydantic_model_creator multiple times with different arguments without specifying a name might return the same model due to caching (_MODEL_INDEX in creator.py). The example below demonstrates the issue.

from tortoise import fields
from tortoise.contrib.pydantic import pydantic_model_creator
from tortoise.models import Model


class People(Model):
    id = fields.IntField(pk=True)
    created_at = fields.DatetimeField(auto_now_add=True)
    modified_at = fields.DatetimeField(auto_now=True)

    first_name = fields.CharField(max_length=50)
    last_name = fields.CharField(max_length=50)


Person = pydantic_model_creator(People)
PersonIn = pydantic_model_creator(People, exclude_readonly=True)

print(list(Person.model_fields))  
# [id, created_at, modified_at, first_name_last_name]
print(list(PersonIn.model_fields))  
# [id, created_at, modified_at, first_name_last_name] 
# However, it should be ['first_name', 'last_name'] because of exclude_readonly=True

print(Person is PersonIn)  
# True (!)

This issues affects the models that have no backward or forward reference to any other model.

This PR:

• Makes pydantic_model_creator to create verbatim names for "root" models even if they do not have relations
• Adds the exclude_readonly to the hash used for naming unnamed models so pydantic_model_creator(People) and pydantic_model_creator(People, exclude_readonly=True)

Motivation and Context

The issues that will be fixed by this PR:

• Closes 'exclude_readonly' is ignored when the 'name' argument isn't used in pydantic_model_creator #594
• Closes got wrong pydantic model when use pydantic_model_creator without specify name #647

There was an attempt to solve this issue previously but it was never merged. The approach used does not seem to work anymore.

How Has This Been Tested?

• Tested the snippets from the issues above
• Added unit tests

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[henadzit]

Hey @waketzheng @waketzheng, any chance you can review this PR? Please let me know if more information or more tests are required! Thanks!