How to pass a table-valued parameter to sp_execute_external_script?
Focusing on Table-Valued Parameters (TVPs) is a bit misleading since the question isn't really concerned with them. The example code, in fact, isn't even using the TVP as a TVP, and would be the same question even if presented as asking about table variables. Meaning, the following adaptation of the OP's example code is really the same question since sp_execute_external_script is being passed a string containing a query and not the @inputTvp variable:

DECLARE @inputTableVariable TABLE
(
  [ID] INT NOT NULL IDENTITY(1, 1) PRIMARY KEY,
  [SomeValue] NVARCHAR(50) NOT NULL
);


DECLARE @tSqlString NVARCHAR(100) = N'SELECT * FROM @inputTableVariable;';


EXEC sp_execute_external_script
    @language = N'Java',
    @script = N'com.example.TestTvp',
    @input_data_1 = @tSqlString; -- this doesn't work


Keep in mind that TVPs are meant to send structured data into SQL Server, not out of it. It's also a variable, hence it would only be available to the local context (i.e. not Dynamic SQL or any other sub-process). Still, just to be sure, I tried the following (in SQL Server 2017 so I couldn't specify "Java" as the language):

EXEC sp_execute_external_script
    @language = N'Java',
    @script = N'com.example.TestTvp',
    @params = N'@inputTableVariable dbo.[testTVP] READONLY';


and received:


  Msg 39018, Level 16, State 2, Line XXXXX
  Parameter '@inputTableVariable' uses a data type that is not supported by the runtime for 'R' script. Unsupported types are timestamp, datetime2, datetimeoffset, time, text, ntext, image, hierarchyid, xml, sql_variant and user-defined type.




That being said, you can pass in structured data (avoiding the use of temporary tables), you just need to serialize it as NVARCHAR(MAX) and pass it as a regular parameter (i.e. not an @input_data_1 query). For serialization, you can use either XML or JSON:

Via XML

DECLARE @XmlString NVARCHAR(MAX);

SET @XmlString = (SELECT *
                  FROM   @inputTvp
                  FOR XML AUTO, ROOT('data'));

PRINT @XmlString; -- DEBUG (else comment out)


EXEC sp_execute_external_script
    @language = N'Java',
    @script = N'com.example.TestTvp',
    @params = N'@inputData NVARCHAR(MAX)',
    @inputData = @XmlString;


Via JSON

DECLARE @JsonString NVARCHAR(MAX);

SET @JsonString = (SELECT *
                   FROM   @inputTvp
                   FOR JSON AUTO, ROOT('data'));

PRINT @JsonString; -- DEBUG (else comment out)

EXEC sp_execute_external_script
    @language = N'Java',
    @script = N'com.example.TestTvp',
    @params = N'@inputData NVARCHAR(MAX)',
    @inputData = @JsonString;