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**Apache Airflow version**:


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

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**: Docker

**What happened**:

When passing a templatized `dest_gcs` argument to the `S3ToGCSOperator` operator, the DAG fails to import because the constructor attempts to test the validity of the URL before the template has been populated in `execute`.

The error is:

```
Broken DAG: [/opt/airflow/dags/bad_gs_dag.py] Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/providers/google/cloud/hooks/gcs.py", line 1051, in gcs_object_is_directory
    _, blob = _parse_gcs_url(bucket)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/providers/google/cloud/hooks/gcs.py", line 1063, in _parse_gcs_url
    raise AirflowException('Please provide a bucket name')
airflow.exceptions.AirflowException: Please provide a bucket name
```

**What you expected to happen**:

The DAG should successfully parse when using a templatized `dest_gcs` value.

**How to reproduce it**:

Instantiating a `S3ToGCSOperator` task with `dest_gcs="{{ var.gcs_url }}"` fails. 

<details>

```python
from airflow.decorators import dag
from airflow.utils.dates import days_ago
from airflow.providers.google.cloud.transfers.s3_to_gcs import S3ToGCSOperator


@dag(
    schedule_interval=None,
    description="Demo S3-to-GS Bug",
    catchup=False,
    start_date=days_ago(1),
)
def demo_bug():

    S3ToGCSOperator(
        task_id="transfer_task",
        bucket="example_bucket",
        prefix="fake/prefix",
        dest_gcs="{{ var.gcs_url }}",
    )


demo_dag = demo_bug()
```
</details>


**Anything else we need to know**:

Should be fixable by moving the code that evaluates whether the URL is a folder to `execute()`.

airflow/providers/google/cloud/transfers/s3_to_gcs.py
:param dest_gcs_conn_id: (Deprecated) The connection ID used to connect to Google Cloud.
        This parameter has been deprecated. You should pass the gcp_conn_id parameter instead.
    :type dest_gcs_conn_id: str
    :param dest_gcs: The destination Google Cloud Storage bucket and prefix
        where you want to store the files. (templated)
    :type dest_gcs: str
    :param delegate_to: Google account to impersonate using domain-wide delegation of authority,
        if any. For this to work, the service account making the request must have
        domain-wide delegation enabled.
    :type delegate_to: str
    :param replace: Whether you want to replace existing destination files
        or not.
    :type replace: bool
    :param gzip: Option to compress file for upload
    :type gzip: bool
    :param google_impersonation_chain: Optional Google service account to impersonate using
        short-term credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.
        If set as a string, the account must grant the originating account
        the Service Account Token Creator IAM role.
        If set as a sequence, the identities from the list must grant
        Service Account Token Creator IAM role to the directly preceding identity, with first
        account from the list granting this role to the originating account (templated).
    :type google_impersonation_chain: Union[str, Sequence[str]]
    **Example**:
    .. code-block:: python
       s3_to_gcs_op = S3ToGCSOperator(
           task_id="s3_to_gcs_example",
           bucket="my-s3-bucket",
           prefix="data/customers-201804",