Fix types / docstrings on DeviceMetadata

[tanujkhattar]

Description of the issue

1. Args say qubits and nx_graph are optional, but their type is not Optional[].
   

   Cirq/cirq-core/cirq/devices/device.py

   Lines 224 to 231 in 6937e41

   	qubits: Iterable['cirq.Qid'],
   	nx_graph: 'nx.graph',
   	):
   	"""Construct a DeviceMetadata object.
   	Args:
   	qubits: Optional iterable of `cirq.Qid`s that exist on the device.
   	nx_graph: Optional `nx.Graph` describing qubit connectivity

2. Args say the returned qubit set can be None but return type is not Optional[FrozenSet['cirq.Qid']].
   

   Cirq/cirq-core/cirq/devices/device.py

   Lines 240 to 245 in 6937e41

   	def qubit_set(self) -> FrozenSet['cirq.Qid']:
   	"""Returns a set of qubits on the device, if possible.
   	Returns:
   	Frozenset of qubits on device if specified, otherwise None.
   	"""

3. Args say the returned graph can be None but return type is not Optional[nx.Graph].
   

   Cirq/cirq-core/cirq/devices/device.py

   Lines 249 to 253 in 6937e41

   	def nx_graph(self) -> 'nx.Graph':
   	"""Returns a nx.Graph where nodes are qubits and edges are couple-able qubits.
   	Returns:
   	`nx.Graph` of device connectivity if specified, otherwise None.

4. nx.graph is used as a type instead of nx.Graph -- the former is a module and latter is a type.
   

   Cirq/cirq-core/cirq/devices/device.py

   Line 225 in 6937e41

   nx_graph: 'nx.graph',

Cirq version

0.14dev

cc @MichaelBroughton