BALANCE SHEET COMPONENTS - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 52,356	$ 52,563
Less: accumulated depreciation and amortization	(32,019)	(28,501)
Property and equipment, net	20,337	24,062
Prototype
Property, Plant and Equipment [Line Items]
Property and equipment, gross	17,730	17,711
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	17,714	17,486
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,041	14,196
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,295	1,295
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,389	1,389
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 187	$ 486