Revert ReplicaType to string

[zw0610]

Since PR #163 has been stalled for a while, this pr tries to accomplish the same goal: revert rtype from commonv1.ReplicaType to string.

For any parameters, if it is of type commonv1.ReplicaType, it's reverted to string in this pr. However, parameters of type map[commonv1.ReplicaType]{any_value_type} remain unchanged.

@Jeffwan

[gaocegege]

LGTM

[terrytangyuan]

/assign @Jeffwan

[zw0610]

@Jeffwan As tested in this pr kubeflow/training-operator#1462, the updated common library works with training-operator.

[gaocegege]

LGTM

[Jeffwan]

This PR can move forward once the issue here kubeflow/training-operator#1462 is addressed.

[zw0610]

ping @Jeffwan @terrytangyuan as the all tested passed: kubeflow/training-operator#1462

However, please note that tf-controller is actually modified to keep up with the labels in new kubeflow/common. I submitted another pr kubeflow/training-operator#1474 to fix the label issue.

[gaocegege]

/lgtm

[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

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