
**Apache Airflow version**: 2.0.1


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): NA

**Environment**: Windows WSL2 (Ubuntu) Local

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04
- **Kernel** (e.g. `uname -a`): Linux d255bce4dcd5 5.4.72-microsoft-standard-WSL2
- **Install tools**: Docker -compose
- **Others**:

**What happened**:
I have a dag with tasks:
Task1 - Get Date
Task 2 - Get data from Api call (Have set retires to 3)
Task 3 - Load Data

Task 2 had failed after three attempts. I am unable to clear the task Instance and get the below error in UI. 

[Dag Code](https://github.com/anilkulkarni87/airflow-docker/blob/master/dags/covidNyDaily.py)

```
Python version: 3.8.7
Airflow version: 2.0.1rc2
Node: d255bce4dcd5
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1547, in clear
    return self._clear_dag_tis(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1475, in _clear_dag_tis
    count = dag.clear(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 1324, in clear
    clear_task_instances(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 160, in clear_task_instances
    ti.max_tries = ti.try_number + task_retries - 1
TypeError: unsupported operand type(s) for +: 'int' and 'str'
```

**What you expected to happen**:

I expected to clear the Task Instance so that the task could be scheduled again.

**How to reproduce it**:
1) Clone the repo link shared above
2) Follow instructions to setup cluster.
3) Change code to enforce error in Task 2 
4) Execute and try to clear task instance after three attempts.

![Error pops up when clicked on Clear](https://user-images.githubusercontent.com/10644132/107998258-8e1ee180-6f99-11eb-8442-0c0be5b23478.png)







airflow/models/baseoperator.py
self,
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        ignore_first_depends_on_past: bool = True,
        ignore_ti_state: bool = False,
        mark_success: bool = False,
    ) -> None:
        """Run a set of task instances for a date range."""
        start_date = start_date or self.start_date
        end_date = end_date or self.end_date or timezone.utcnow()
        for execution_date in self.dag.date_range(start_date, end_date=end_date):
            TaskInstance(self, execution_date).run(
                mark_success=mark_success,
                ignore_depends_on_past=(execution_date == start_date and ignore_first_depends_on_past),
                ignore_ti_state=ignore_ti_state,
            )
    def dry_run(self) -> None: