**Apache Airflow version**: 2.0.1


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): 1.15

**Environment**:

- **Cloud provider or hardware configuration**: k8s on bare metal
- **OS** (e.g. from /etc/os-release): 
- **Kernel** (e.g. `uname -a`):
- **Install tools**: pip3
- **Others**:

**What happened**:
Scheduler dies with 
```
[2021-02-10 21:09:27,469] {scheduler_job.py:1298} ERROR - Exception when executing SchedulerJob._run_schedu
ler_loop
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1280, in _execute
    self._run_scheduler_loop()
  File "/usr/local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1384, in _run_scheduler
_loop
    self.executor.heartbeat()
  File "/usr/local/lib/python3.8/site-packages/airflow/executors/base_executor.py", line 158, in heartbeat
    self.trigger_tasks(open_slots)
  File "/usr/local/lib/python3.8/site-packages/airflow/executors/base_executor.py", line 188, in trigger_ta
sks
    self.execute_async(key=key, command=command, queue=None, executor_config=ti.executor_config)
  File "/usr/local/lib/python3.8/site-packages/airflow/executors/kubernetes_executor.py", line 493, in exec
ute_async
    kube_executor_config = PodGenerator.from_obj(executor_config)
  File "/usr/local/lib/python3.8/site-packages/airflow/kubernetes/pod_generator.py", line 175, in from_obj
    k8s_legacy_object = obj.get("KubernetesExecutor", None)
AttributeError: 'V1Pod' object has no attribute 'get'
[2021-02-10 21:09:28,475] {process_utils.py:100} INFO - Sending Signals.SIGTERM to GPID 60
[2021-02-10 21:09:29,222] {process_utils.py:66} INFO - Process psutil.Process(pid=66, status='terminated',
started='21:09:27') (66) terminated with exit code None
[2021-02-10 21:09:29,697] {process_utils.py:206} INFO - Waiting up to 5 seconds for processes to exit...
[2021-02-10 21:09:29,716] {process_utils.py:66} INFO - Process psutil.Process(pid=75, status='terminated',
started='21:09:28') (75) terminated with exit code None
[2021-02-10 21:09:29,717] {process_utils.py:66} INFO - Process psutil.Process(pid=60, status='terminated',
exitcode=0, started='21:09:27') (60) terminated with exit code 0
[2021-02-10 21:09:29,717] {scheduler_job.py:1301} INFO - Exited execute loop
```

**What you expected to happen**:
DAG loading fails, producing an error for just that DAG, instead of crashing the scheduler.

**How to reproduce it**:
Create a task like
```
    test = DummyOperator(task_id="new-pod-spec",
                              executor_config=k8s.V1Pod(
                                      spec=k8s.V1PodSpec(
                                          containers=[
                                              k8s.V1Container(
                                                  name="base",
                                                  image="myimage",
                                                  image_pull_policy="Always"
                                              )
                                          ]
                                      )))
```
or 
```
    test = DummyOperator(task_id="new-pod-spec",
                              executor_config={"KubernetesExecutor": k8s.V1Pod(
                                      spec=k8s.V1PodSpec(
                                          containers=[
                                              k8s.V1Container(
                                                  name="base",
                                                  image="myimage",
                                                  image_pull_policy="Always"
                                              )
                                          ]
                                      ))})
```
essentially anything where it expects a dict but gets something else, and run the scheduler using the kubernetes executor

tests/executors/test_kubernetes_executor.py
#
#
#
import random
import re
import string
import unittest
from datetime import datetime
from unittest import mock
from kubernetes.client import models as k8s
from urllib3 import HTTPResponse
from airflow.utils import timezone
from tests.test_utils.config import conf_vars
try:
    from kubernetes.client.rest import ApiException
    from airflow.executors.kubernetes_executor import (
        AirflowKubernetesScheduler,
        KubernetesExecutor,
        create_pod_id,
        get_base_pod_from_template,
    )
    from airflow.kubernetes import pod_generator
    from airflow.kubernetes.pod_generator import PodGenerator
    from airflow.utils.state import State
except ImportError:
    AirflowKubernetesScheduler = None  
class TestAirflowKubernetesScheduler(unittest.TestCase):